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

[Tom Stellard via llvm-commits]

================
@@ -15,7 +19,7 @@ if(LLVM_BUILD_INSTRUMENTED)
     )
----------------
tstellar wrote:

@petrhosek CLANG_PERF_TRAINING_DEPS was an existing variable that's used here.  I think it was meant to be added to cache files, but I added it to CMakeLists.txt file here so that it could be passed on the command line without modifying the cache files.

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