[llvm] [lit][NFC] Refactor use_clang into two functions (PR #147436)

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> Could you do a stacked patch to show how this helps clang-tools-extra? Presumably you've tried it to verify the fix manually.

It's https://github.com/llvm/llvm-project/pull/147437. I put it up but forgot to link the two explicitly in the comments.

https://github.com/llvm/llvm-project/pull/147436