[PATCH] Make error handling functions thread-safe.

[Chandler Carruth]

Looks good with a slightly relaxed comment.

================
Comment at: lib/Support/ErrorHandling.cpp:74-77
@@ +73,6 @@
+  {
+    // The LLVM error handler is likely to call exit(), which should not be done
+    // under a mutex (some mutex implementations exhibit undefined behavior if
+    // destroyed while acquired).  So the mutex is acquired only while reading
+    // the error handler.
+    llvm::MutexGuard Lock(ErrorHandlerMutex);
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This comment is a bit too specific IMO. It is never a good idea to call a user provided callback while holding a mutex.

http://reviews.llvm.org/D4140