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

[Tom Stellard via cfe-commits]

tstellar 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?

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