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

[Wael Yehia via cfe-commits]

w2yehia wrote:

Follow up PR https://github.com/w2yehia/llvm-project/pull/1 to switch all applicable tests in `compiler-rt/test/profile/ContinuousSyncMode` to use `-fprofile-continuous`

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