[llvm] Add thread-local overrides for `llvm::errs()` and `llvm::outs()`. (PR #90374)

[Chandler Carruth via llvm-commits]

https://github.com/chandlerc commented:

Sorry for crossing reviews, but also thanks for the reviews!

> I agree with David it would be nice if these just weren't needed, but I also know of quite a few places in llvm and clang that just grab outs/errs. I'm fine with this approach if we're sure it's not easy to make it so they can be passed in.

Yeah, I really wish I saw an easy way to solve the issues I'm hitting, see the previous update. The proximate thing I'm doing is trying to use the Clang Driver as a library and pretty quickly hit these w/o a lot of great ideas of how to thread things through. If folks see a way I missed or disagree, lemme know though.

Also, responses to inline comments below.

https://github.com/llvm/llvm-project/pull/90374