[llvm][macos] Fix usage of std::shared_mutex on old macOS SDK versions

[thieta]

Summary

When setting CMAKE_CXX_STANDARD to 17 and targeting a macOS version
under 10.12 the ifdefs would try to use std::shared_mutex because
the of the C++ standard. This should also check the targeted SDK.

See discussion in: https://reviews.llvm.org/D130689

[nikic]

LG

[thieta]

Ping @thakis

llvm/include/llvm/Support/RWMutex.h

 #endif
 
 /// SmartMutex - An R/W mutex with a compile time constant parameter that
 /// indicates whether this mutex should become a no-op when we're not
 /// running in multithreaded mode.
 template <bool mt_only> class SmartRWMutex {
   // shared_mutex (C++17) is more efficient than shared_timed_mutex (C++14)
   // on Windows and always available on MSVC except with libc++.
+#if !defined(LLVM_USE_RW_MUTEX_IMPL)
 #if (defined(_MSC_VER) && !defined(_LIBCPP_VERSION)) || __cplusplus > 201402L
   std::shared_mutex impl;
 #else
-#if !defined(LLVM_USE_RW_MUTEX_IMPL)
   std::shared_timed_mutex impl;
+#endif
 #else
   RWMutexImpl impl;
 #endif
-#endif
   unsigned readers = 0;
   unsigned writers = 0;
 
 public:
   bool lock_shared() {
     if (!mt_only || llvm_is_multithreaded()) {
       impl.lock_shared();
       return true;