Always use pthread_rwlock on Apple platforms (#70151)

pthread_rwlock is faster than std::shared_mutex, especially in heavily
threaded code such as the Swift compiler.

rdar://117445844

llvm/include/llvm/Support/RWMutex.h

@@ -19,12 +19,9 @@
 #include <mutex>
 #include <shared_mutex>
 
-// std::shared_timed_mutex is only available on macOS 10.12 and later.
-#if defined(__APPLE__) && defined(__ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__)
-#if __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ < 101200
+#if defined(__APPLE__)
 #define LLVM_USE_RW_MUTEX_IMPL
 #endif
-#endif
 
 namespace llvm {
 namespace sys {