[llvm] Add back Allocator.h include to RWMutex.cpp.

This unbreaks the build on macOS.

Without the include, the build fails with

llvm/lib/Support/RWMutex.cpp:47:36: error: use of undeclared identifier 'safe_malloc'
   47 |
   static_cast<pthread_rwlock_t*>(safe_malloc(sizeof(pthread_rwlock_t)));
         |                                    ^

Author: fhahn

llvm/lib/Support/RWMutex.cpp

@@ -13,6 +13,7 @@
 #include "llvm/Support/RWMutex.h"
 #include "llvm/Config/config.h"
 #include "llvm/Config/llvm-config.h" // for LLVM_ENABLE_THREADS
+#include "llvm/Support/Allocator.h"
 
 #if defined(LLVM_USE_RW_MUTEX_IMPL)
 using namespace llvm;