[llvm] [llvm-profgen][SPGO] Support profiles with multiple concurrent processes (PR #169353)

[via llvm-commits]

https://github.com/HighW4y2H3ll commented:

Also just saw this code: https://github.com/llvm/llvm-project/blob/7c9c6bec15203f04f4fe06f0a1926117a7f5a789/llvm/tools/llvm-profgen/PerfReader.cpp#L474-L499 So maybe we don't really need to introduce extra cmdline args afterall?

https://github.com/llvm/llvm-project/pull/169353