Build system tweaks patchset

[ChrisCummins]

This pull request makes two important changes to the build:

• Fixes a bunch of errors that were introduced in Remove build-time codegen using Python #620 (not sure what happened there), un-breaking the build.
• Adds support for building CompilerGym against a pre-compiled LLVM. This reduces CI build time by 35 minutes, and reduces the size of the build directory by 55% (11 GB to 4.9 GB) (Speedup CMake Configuration #595).

And a bunch of smaller tweaks to the CMake build:

• Switches the CI cmake job to use a pre-compiled LLVM.
• Updates the version of absl and benchmark to work around some build errors on newer linux releases.
• Updates ProGraML so that it uses the same version of absl.
• Turns off the redundant conditional logic for building on Darwin. There is no Linux/macOS-dependent logic in the build system (except for janky linker workarounds needed by bazel).
• Automatically enables ccache in the CMake config when it is available.
• Automatically uses the lld linker when it is available.
• Forces the Ninja generator.
• Makes a number of small formatting tweaks for readability.

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My eventual goal is to add support for CMake on macOS (#618), but I am hitting a problem with the protobuf configuration step (autoreconf) in external/protobuf/build_protobuf.cmake:

checking for g++ options needed to detect all undeclared functions... cannot detect
configure: error: in `/Users/cummins/src/CompilerGym/build/external/protobuf/protobuf/src/protobuf':
configure: error: cannot make g++ report undeclared builtins
See `config.log' for more details
CMake Error at /Users/cummins/src/CompilerGym/external/protobuf/build_protobuf.cmake:50 (execute_process):
  execute_process failed command indexes:

    1: "Child return code: 1"

For some reason, the configure script doesn't like the detected C/C++ compiler. I'll pick this up at a later date.

[codecov-commenter]

Codecov Report

Merging #618 (26e287d) into development (4ca7f19) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           development     #618      +/-   ##
===============================================
- Coverage        88.60%   88.54%   -0.06%     
===============================================
  Files              115      115              
  Lines             7019     7019              
===============================================
- Hits              6219     6215       -4     
- Misses             800      804       +4     

Impacted Files	Coverage Δ
...er_gym/third_party/inst2vec/inst2vec_preprocess.py	69.73% <0.00%> (-4.22%)	⬇️
compiler_gym/util/commands.py	83.33% <0.00%> (-2.39%)	⬇️
compiler_gym/envs/compiler_env.py	92.34% <0.00%> (+0.49%)	⬆️
compiler_gym/views/observation.py	100.00% <0.00%> (+2.70%)	⬆️
...loop_tool/service/loop_tool_compilation_session.py	90.47% <0.00%> (+2.72%)	⬆️
compiler_gym/views/observation_space_spec.py	89.28% <0.00%> (+3.57%)	⬆️

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[ChrisCummins]

@sogartar could you take a quick look at this?

Cheers,
Chris

[ChrisCummins]

Merging now as this fixes the development build.

Cheers,
Chris