[clang] [llvm] [Clang][CMake] Add CSSPGO support to LLVM_BUILD_INSTRUMENTED (PR #79942)

[Amir Ayupov via cfe-commits]

aaupov wrote:

Ping @boomanaiden154, @WenleiHe, @wlei-llvm.

I've updated the diff to use external training set (CLANG_PGO_TRAINING_DATA_SOURCE_DIR), and included performance testing results.

https://github.com/llvm/llvm-project/pull/79942