[llvm] [clang] [CMake][PGO] Add option for using an external project to generate profile data (PR #78879)

[Amir Ayupov via cfe-commits]

aaupov wrote:

I'm in favor of this change especially in context of CSSPGO profiling (https://github.com/llvm/llvm-project/pull/79942) and BOLT LBR profiling (https://github.com/llvm/llvm-project/pull/69133).

https://github.com/llvm/llvm-project/pull/78879