fix: gcc uninitialized warnings triggered by unsafe_default_initialized

Despite all the `FOLLY_*_DISABLE_WARNING`, the implementation of
`unsafe_default_initialized_cv` triggers hundreds of `-Wuninitialized`
and `-Wmaybe-uninitialized` warnings when building with GCC 12, 13, 14.
As much as I've tried, it seems impossible to silence the warnings
purely by diagnostic pragmas.

This change, however, eliminates all warnings, and the new code should
otherwise have the exact same effect as before. At least, I can see
that clang / gcc both produce identical code before and after this
change when using `unsafe_default_initialized` to initialize variables.

folly/Utility.h

@@ -434,8 +434,8 @@ struct unsafe_default_initialized_cv {
 #if __cpp_lib_is_constant_evaluated >= 201811L
 #if !defined(__MSVC_RUNTIME_CHECKS)
     if (!std::is_constant_evaluated()) {
-      T uninit;
-      return uninit;
+      alignas(T) char uninit[sizeof(T)];
+      return *reinterpret_cast<T*>(uninit);
     }
 #endif // !defined(__MSVC_RUNTIME_CHECKS)
 #endif