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

[Alexandre Ganea via llvm-commits]

================
@@ -893,7 +893,13 @@ void raw_fd_ostream::anchor() {}
 //  outs(), errs(), nulls()
 //===----------------------------------------------------------------------===//
 
+static LLVM_THREAD_LOCAL raw_fd_ostream *OutsOverride = nullptr;
+static LLVM_THREAD_LOCAL raw_fd_ostream *ErrsOverride = nullptr;
+
 raw_fd_ostream &llvm::outs() {
+  if (auto *TLSOuts = OutsOverride; TLSOuts != nullptr)
----------------
aganea wrote:

Similarly here, `TLSOuts != nullptr` is implied, we could just say:
```
if (auto *TLSOuts = OutsOverride)
  return *TLSOuts;
```

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