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

[Chandler Carruth via llvm-commits]

chandlerc wrote:

One thing to mention is that I'm open to other suggestions here. For example, if folks want it could be put behind a build configuration macro so it's only used in cases where it is needed. Somewhat up to others how they'd like this to work, I'm mostly just trying to find a way to send tool output from libraries to controlled streams.

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