Adding llvm::deleteManagedStaticMutex

[Anastasia]

I think this might be for a discussion on llvm-dev with a good example of how it should be used. Otherwise, the purpose of this API is not that clear.

include/llvm/Support/ManagedStatic.h

 /// llvm_shutdown - Deallocate and destroy all ManagedStatic variables.
 void llvm_shutdown();
 
+/// Purpose of this function is to free memory allocated for ManagedStaticMutex.
+/// One might want to do that to avoid memory leaks in case LLVM is loaded as a
+/// shared library (or dll) at runtime.
+/// This function is not thread safe and should be called only if there are no
+/// threads which are using the mutex now or will use the mutex in the future.
+/// This means deleteManagedStaticMutex can be called only after llvm_shutdown.
+void deleteManagedStaticMutex();
+
 /// llvm_shutdown_obj - This is a simple helper class that calls
 /// llvm_shutdown() when it is destroyed.
 struct llvm_shutdown_obj {

lib/Support/ManagedStatic.cpp

   while (StaticList)
     StaticList->destroy();
 }
+
+void llvm::deleteManagedStaticMutex() {
+  assert(StaticList == nullptr && "llvm_shutdown() must be called first!");
+  delete ManagedStaticMutex;
+}
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