Fix compilation error with -DLIBCXXABI_ENABLE_THREADS=OFF

[michaelplatings]

Summary

The error is:

libcxxabi/src/cxa_guard_impl.h: In instantiation of ‘__cxxabiv1::{anonymous}::LibcppMutex __cxxabiv1::{anonymous}::GlobalStatic<__cxxabiv1::{anonymous}::LibcppMutex>::instance’:
/work/llvm/llvm-project/libcxxabi/src/cxa_guard_impl.h:529:62:   required from here
/work/llvm/llvm-project/libcxxabi/src/cxa_guard_impl.h:510:23: error: ‘__cxxabiv1::{anonymous}::LibcppMutex __cxxabiv1::{anonymous}::GlobalStatic<__cxxabiv1::{anonymous}::LibcppMutex>::instance’ has incomplete type
 _LIBCPP_SAFE_STATIC T GlobalStatic<T>::instance = {};
                       ^

This was introduced in this commit: https://github.com/llvm/llvm-project/commit/70ebeabfb8330f805a26dbb2f8af77fb0ab0c5f0#diff-598857ff93b4283c6978bca09e013e02

[EricWF]

Thanks!

[michaelplatings]

https://github.com/llvm/llvm-project/commit/d144572dac23c3af6a1ea5efbdffd8813138e216

[michaelplatings]

@EricWF this change seems to have broken some a test somehow. I'm well out of my depth here so can you take a look?
http://lab.llvm.org:8011/builders/llvm-clang-lld-x86_64-scei-ps4-windows10pro-fast/builds/25356/steps/test/logs/stdio

libcxxabi/src/cxa_guard_impl.h

struct LibcppCondVar {
   bool wait(LibcppMutex& mut) {
     return std::__libcpp_condvar_wait(&cond, &mut.mutex);
   }
   bool broadcast() { return std::__libcpp_condvar_broadcast(&cond); }
 
 private:
   std::__libcpp_condvar_t cond = _LIBCPP_CONDVAR_INITIALIZER;
 };
+#else
+struct LibcppMutex {};
+struct LibcppCondVar {};
 #endif // !defined(_LIBCXXABI_HAS_NO_THREADS)
 
 
 template <class Mutex, class CondVar, Mutex& global_mutex, CondVar& global_cond,
           uint32_t (*GetThreadID)() = PlatformThreadID>
 struct InitByteGlobalMutex
     : GuardObject<InitByteGlobalMutex<Mutex, CondVar, global_mutex, global_cond,
                                     GetThreadID>> {