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

[Michael Spencer via llvm-commits]

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@@ -893,7 +893,13 @@ void raw_fd_ostream::anchor() {}
 //  outs(), errs(), nulls()
 //===----------------------------------------------------------------------===//
 
+static thread_local raw_fd_ostream *OutsOverride = nullptr;
+static thread_local raw_fd_ostream *ErrsOverride = nullptr;
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Bigcheese wrote:

These should probably use `LLVM_THREAD_LOCAL` so they are just normal globals when `LLVM_ENABLE_THREADS` is false.

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