Build clang package with PGO

[zamazan4ik]

Describe the bug

LLVM supports building Clang with PGO (https://llvm.org/docs/HowToBuildWithPGO.html). Using PGO for compilers has a huge impact on compiler performance. E.g. many distros are already building GCC (which also supports PGO builds) with PGO. I think for the users would be beneficial to have faster Clang binaries.

Locally I usually build my own Clang version. According to my tests, it could bring up to 20% performance.

Additionally, you could consider using LLVM BOLT as an additional optimization step, but I guess it should be discussed in another issue after the PGO implementation.

Expected behavior

clang package will be PGO-optimized.

Other notes

It's not actually a bug - just an improvement request. Feel free to assign to the more proper category.

[tpwrules]

FWIW, the community explicitly made the decision to turn off PGO in GCC in order to have proper reproducibility: #112928