ADT: Add SFINAE to the generic IntrusiveRefCntPtr constructors

Add an `enable_if` to the generic `IntrusiveRefCntPtr` constructors so
that std::is_convertible gives an honest answer when the underlying
pointers cannot be converted. Added `static_assert`s to the test suite
to verify.

Also combine generic constructors from `IntrusiveRefCntPtr<X>&&` and
`const IntrusiveRefCntPtr<X>&`. At first glance this appears to be an
infinite loop, but the real copy/move constructors are spelled out
separately above. Added a unit test to verify.

Differential Revision: https://reviews.llvm.org/D95498