[WIP] use built-in .NET runtime types instead of vendored types when possible

[lucaspimentel]

Summary of changes

When targeting .NET Core >= 3.1, do not include the files copied ("vendored") from the .NET runtime. Instead, use the built-in types.

Reason for change

• main goal is smaller artifacts for .NET Core 3.1 and .NET 6.0 (important for Serverless)
• secondary goal is consistency in our code base: we currently have different ways of choosing built-in types vs vendored types: sometimes we always use the vendored type, sometimes we choose based on target runtime (TFM)

Implementation details

• Remove most references to these namespaces from individual files.
• In GlobalUsings.cs, use #if NETCOREAPP3_1_OR_GREATER and global using to choose namespaces based on target runtime (TFM)
• Replace the use of some internal .NET runtime APIs with their public counterparts, for example:
  • ArrayMemoryPool ➡️ MemoryPool
  • new ImmutableArray<LocalScope>.Builder() ➡️ ImmutableArray.CreateBuilder<LocalScope>()
• When targeting netcoreapp3.1 or net6.0, exclude most of the files in:

Vendors/System.Collections.Immutable;
Vendors/System.Memory;
Vendors/System.Private.CoreLib;
Vendors/System.Runtime.CompilerServices.Unsafe;

Note that this PR keeps the files in Vendors/System.Reflection.Metadata for now because we access non-public members from that namespace. This namespace makes up about 60% of the bytes in ``Vendors/System.*`. We can tackle this in a future PR.

This PR also keeps a few files which are used elsewhere:

Vendors/System.Private.CoreLib/src/System/Runtime/InteropServices/LibraryImportAttribute.cs
Vendors/System.Private.CoreLib/src/System/Diagnostics/CodeAnalysis/7NullableAttributes.cs
Vendors/System.Private.CoreLib/src/System/Runtime/InteropServices/StringMarshalling.cs
Vendors/System.Private.CoreLib/SR.cs

Test coverage

No behavior changes expected. All tests should still pass.

Other details

Assembly size comparison

Runtime	Before	After	Diff	%
netcoreapp3.1	7.59 MB	7.25 MB	-347 KB	-4.6%
net6.0	7.62 MB	7.28 MB	-346 KB	-4.5%

Estimated size per namespace

Before (net6.0)

VendoredMicrosoftCode 0.65 MB (12.2%)
└── System 0.65 MB (12.2%)
    ├── Reflection 0.4 MB (7.6%)
    ├── Collections 0.12 MB (2.3%)
    ├── Buffers 0.06 MB (1.1%)
    ├── Runtime < 0.01 MB (0.1%)
    ├── Linq < 0.01 MB (< 0.1%)
    ├── Diagnostics < 0.01 MB (< 0.1%)
    └── Memory < 0.01 MB (< 0.1%)

After (net6.0)

VendoredMicrosoftCode 0.42 MB (8.2%)
└── System 0.42 MB (8.2%)
    ├── Reflection 0.4 MB (7.9%)
    └── Runtime < 0.01 MB (< 0.1%)

[datadog-datadog-prod-us1]

Datadog Report

All test runs 0b047b0 🔗

✅ 2 Total Test Services: 0 Failed, 2 Passed

Test Services

Service Name	Failed	Known Flaky	New Flaky	Passed	Skipped	Total Time	Test Service View
dd-trace-dotnet	0	0	0	566125	4562	34h 54m 19.52s	Link
exploration_tests	0	0	0	22085	3	1h 28m 49.47s	Link

[andrewlock]

Execution-Time Benchmarks Report ⏱️

Execution-time results for samples comparing the following branches/commits:

• This PR (6726)
• master

Execution-time benchmarks measure the whole time it takes to execute a program. And are intended to measure the one-off costs. Cases where the execution time results for the PR are worse than latest master results are shown in red. The following thresholds were used for comparing the execution times:

• Welch test with statistical test for significance of 5%
• Only results indicating a difference greater than 5% and 5 ms are considered.

Note that these results are based on a single point-in-time result for each branch. For full results, see the dashboard.

Graphs show the p99 interval based on the mean and StdDev of the test run, as well as the mean value of the run (shown as a diamond below the graph).

gantt
    title Execution time (ms) FakeDbCommand (.NET Framework 4.6.2) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (6726) - mean (69ms)  : 66, 72
     .   : milestone, 69,
    master - mean (70ms)  : 65, 74
     .   : milestone, 70,

    section CallTarget+Inlining+NGEN
    This PR (6726) - mean (1,002ms)  : 979, 1025
     .   : milestone, 1002,
    master - mean (1,008ms)  : 978, 1037
     .   : milestone, 1008,

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gantt
    title Execution time (ms) FakeDbCommand (.NET Core 3.1) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (6726) - mean (103ms)  : 100, 105
     .   : milestone, 103,
    master - mean (103ms)  : 101, 106
     .   : milestone, 103,

    section CallTarget+Inlining+NGEN
    This PR (6726) - mean (675ms)  : 658, 692
     .   : milestone, 675,
    master - mean (691ms)  : 675, 707
     .   : milestone, 691,

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gantt
    title Execution time (ms) FakeDbCommand (.NET 6) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (6726) - mean (89ms)  : 88, 91
     .   : milestone, 89,
    master - mean (89ms)  : 88, 91
     .   : milestone, 89,

    section CallTarget+Inlining+NGEN
    This PR (6726) - mean (633ms)  : 620, 646
     .   : milestone, 633,
    master - mean (648ms)  : 634, 663
     .   : milestone, 648,

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gantt
    title Execution time (ms) HttpMessageHandler (.NET Framework 4.6.2) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (6726) - mean (191ms)  : 187, 196
     .   : milestone, 191,
    master - mean (191ms)  : 186, 195
     .   : milestone, 191,

    section CallTarget+Inlining+NGEN
    This PR (6726) - mean (1,108ms)  : 1078, 1139
     .   : milestone, 1108,
    master - mean (1,107ms)  : 1081, 1132
     .   : milestone, 1107,

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gantt
    title Execution time (ms) HttpMessageHandler (.NET Core 3.1) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (6726) - mean (271ms)  : 266, 276
     .   : milestone, 271,
    master - mean (271ms)  : 267, 276
     .   : milestone, 271,

    section CallTarget+Inlining+NGEN
    This PR (6726) - mean (859ms)  : 833, 885
     .   : milestone, 859,
    master - mean (875ms)  : 843, 907
     .   : milestone, 875,

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gantt
    title Execution time (ms) HttpMessageHandler (.NET 6) 
    dateFormat  X
    axisFormat %s
    todayMarker off
    section Baseline
    This PR (6726) - mean (262ms)  : 259, 265
     .   : milestone, 262,
    master - mean (263ms)  : 259, 267
     .   : milestone, 263,

    section CallTarget+Inlining+NGEN
    This PR (6726) - mean (846ms)  : 809, 883
     .   : milestone, 846,
    master - mean (854ms)  : 826, 882
     .   : milestone, 854,

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