[CMake] Add TEST_SUITE_SAMPLE_PROFILE_USE for sample-based PGO

[mustartt]

Add support for sample profile support (-fprofile-sample-use) to the test suite with two new CMake flags:

• TEST_SUITE_SAMPLE_PROFILE_USE=On|Off: enables sample profile use.
• TEST_SUITE_SAMPLE_PROFILE_DIR=<path>: a directory containing <target>.spgo sample profile files.

Unlike instrumentation PGO which requires a two-phase generate + use workflow, sample profiles are collected externally, and the collection method various widely between platforms. For each benchmark target, the build system looks for <dir>/<target>.spgo and passes -fprofile-sample-use=<path> to the compile flags. In case, the target does not have an associated sample profile, an warning is emitted.

# collection
cmake -DTEST_SUITE_BENCHMARKING_ONLY=On \
      -DTEST_SUITE_RUN_UNDER=<collection tool> \
      ../llvm-test-suite
make 
llvm-lit .

# merge sample profiles into /path/to/profiles
/path/to/profiles/ contains:
401.bzip2.spgo
429.mcf.spgo
456.hmmer.spgo
470.lbm.spgo
...

# final build
cmake -DTEST_SUITE_BENCHMARKING_ONLY=On \
      -DTEST_SUITE_SAMPLE_PROFILE_USE=On \
      -DTEST_SUITE_SAMPLE_PROFILE_DIR=/path/to/profiles \
      ../llvm-test-suite
...