PR19668, PR23034: Fix handling of move constructors and deleted copy
constructors when deciding whether classes should be passed indirectly.
This fixes ABI differences between Clang and GCC:
- Previously, Clang ignored the move constructor when making this determination. It now takes the move constructor into account, per https://github.com/itanium-cxx-abi/cxx-abi/pull/17 (this change may seem recent, but the ABI change was agreed on the Itanium C++ ABI list a long time ago).
- Previously, Clang's behavior when the copy constructor was deleted was unstable -- depending on whether the lazy declaration of the copy constructor had been triggered, you might get different behavior. We now eagerly declare the copy constructor whenever its deletedness is unclear, and ignore deleted copy/move constructors when looking for a trivial such constructor.
This also fixes an ABI difference between Clang and MSVC:
- If the copy constructor would be implicitly deleted (but has not been lazily declared yet), for instance because the class has an rvalue reference member, we would pass it directly. We now pass such a class indirectly, matching MSVC.