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

[Petr Hosek via cfe-commits]

petrhosek wrote:

@tstellar We're trying to build a more minimal corpus that would address the issues I listed with @ilovepi. In the meantime, it'd be helpful to have this option since the current implementation actually breaks on our builders (which don't have host toolchain and sysroot installed).

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