[clang] [PGO] Add a clang option -fprofile-continuous that enables PGO continuous mode (PR #124353)

[Shafik Yaghmour via cfe-commits]

https://github.com/shafik commented:

Please add a summary to your PR something like `Enables continuous PGO mode where profile counter updates are continuously synced to a file.` seems ok. 

This is what will show up in the git log and it useful to have more than a title in the log.

https://github.com/llvm/llvm-project/pull/124353