[lit][NFC] Refactor use_clang into two functions
This patch refactors use_clang into two functions. This is intended for
use within the clang-tools-extra test suite to avoid a race condition
where the clang binary exists but is not yet ready for execution which
results in a lit configuration failure.
Details are in #145703.
Reviewers: Keenuts, pogo59, AaronBallman, DavidSpickett
Reviewed By: pogo59
Pull Request: https://github.com/llvm/llvm-project/pull/147436
