[clang] [llvm] [CMake][PGO] Build Sema.cpp to generate profdata for PGO builds (PR #77347)

[Tom Stellard via cfe-commits]

tstellar wrote:

I was able to figure out how to generate profile data by compiling Sema.cpp from the clang source tree, so I switched that to the default and added an option so that users can override this with an external CMake project, like the LLVM Test Suite.

https://github.com/llvm/llvm-project/pull/77347