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

[Michael Spencer via llvm-commits]

https://github.com/Bigcheese commented:

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.

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