[clang] [clang][perf-training] Fix profiling with -DCLANG_BOLT=perf (PR #119117)

[Amir Ayupov via cfe-commits]

aaupov wrote:

> @aaupov When we build llvm-support there is one perf.data file generate for each cpp file compiled, so we end up without about 150 files.  Is there some way to merge those together before running perf2bolt?

I see. The best way would be to run perf once so that all clang invocations are under it. If llvm-support is configured as cmake rule for llvm external project, perf wrapper could be set as CMAKE_COMPILER_LAUNCHER.

https://github.com/llvm/llvm-project/pull/119117