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

[Richard Smith via llvm-commits]

https://github.com/zygoloid commented:

I think it would also be important for `llvm::CrashRecoveryContext` to persist the stream overrides into the inner thread.

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