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

[David Blaikie via llvm-commits]

================
@@ -893,7 +893,14 @@ void raw_fd_ostream::anchor() {}
 //  outs(), errs(), nulls()
 //===----------------------------------------------------------------------===//
 
+static thread_local raw_fd_ostream *OutsOverride = nullptr;
+static thread_local raw_fd_ostream *ErrsOverride = nullptr;
+
 raw_fd_ostream &llvm::outs() {
+  if (auto *TLSOuts = OutsOverride; TLSOuts != nullptr) {
+    return *TLSOuts;
+  }
+
----------------
dwblaikie wrote:

I guess this would be more LLVM idiomatic?
```
if (auto *TLSOuts = OutsOverride)
  return *TLSOuts;
```

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