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

[David Blaikie via llvm-commits]

https://github.com/dwblaikie commented:

I worry that a tool like this would disincentivize fixing/improving existing libraries that could/should take a stream as a parameter... - so I'm weakly not in favor of this, in that I'd rather see the library improvements... but no doubt there are some hard cases.

Are the cases you have in mind in-tree/could you give a rough overview of the challenges/why this is the better path?

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