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

[Konrad Kleine via cfe-commits]

kwk wrote:

@tstellar thank you for looping me in. When I did my experiments I've used the llvm-test-suite and `test-suite/utils/compare.py --metric exec_time --metric compile_time --metric link_time --lhs-name 16.0.3 --rhs-name 16.0.2-pgo /root/rawhide/results.json vs /root/pgo/results.json` to get the geomean difference. How do you measure performance? Maybe the answer is in your code but I haven't checked your code yet because I am reading this on my phone. 

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