plaintext-backend: export log for other uses

[ZenithalHourlyRate]

Following #1550 (comment), I was able to persist the log file as a target and the openfhe test can see it. However there are some path problems. The file structure heir-opt would see is

./tests/Examples/openfhe/ckks/dot_product_8f_debug/dot_product_8f.mlir
...
./bazel-out/k8-dbg/bin/tools/heir-opt.runfiles/tk_tcl/library/word.tcl
...
./bazel-out/k8-dbg/bin/tests/Examples/openfhe/ckks/dot_product_8f_debug/dot_product_8f_debug.log

Note the bazel-out/k8-dbg. It means that if we want to import some generated file into heir-opt through the data label, its path is ugly.

The argument for the test is

    data = [
        ":dot_product_8f_debug.log",
    ],
    heir_opt_flags = [
        "--mlir-to-ckks=ciphertext-degree=8 encryption-technique-extended=true plaintext-execution-result-file-name=./bazel-out/k8-dbg/bin/tests/Examples/openfhe/ckks/dot_product_8f_debug/dot_product_8f_debug.log",
        "--scheme-to-openfhe=insert-debug-handler-calls=true",
    ],
...


plaintext_test(
    name = "dot_product_8f_debug_plaintext",
    log_file_name = "dot_product_8f_debug.log",

I have not find a way to turn a local label into path.

[ZenithalHourlyRate]

Get it working with manual expansion of location in rule impl

-    args.add_all(ctx.attr.pass_flags)
+    pass_flags_location_expanded = [ctx.expand_location(flag, ctx.attr.data) for flag in ctx.attr.pass_flags]
+    args.add_all(pass_flags_location_expanded)

[ZenithalHourlyRate]

Rebased after #1550 is in.

[ZenithalHourlyRate]

Rebased to resolve merge conflict

[j2kun]

I believe this PR is failing to integrate due to the use of symlinks. Though I know we had used symlinks before, for whatever reason this time it's raising an exception in copybara. I have filed a bug internally, but when I am done with the conference craziness (Monday) I can look into how to migrate this to use bazel filegroup to bring this in line with bazel best practices.

[j2kun]

The bug was fixed internally so we will probably be able to merge this as-is in a day or two.

[asraa]

I can pull it in!