[clang] [clang][perf-training] Support excluding LLVM build from PGO training (PR #126876)

[Tom Stellard via cfe-commits]

tstellar wrote:

@petrhosek I also wanted to mention, if you are interested in upstreaming this minimal corpus, I would support making it the default instead of the current llvm-support build. 

https://github.com/llvm/llvm-project/pull/126876