[clang][Sema] Fix crash on invalid base destructor

LookupSpecialMember might fail, so changes the cast to cast_or_null.
Inside Sema, skip a particular base, similar to other cases, rather than
asserting on dtor showing up.

Other option would be to mark classes with invalid destructors as invalid, but
that seems like a lot more invasive and we do lose lots of diagnostics that
currently work on classes with broken members.

Differential Revision: https://reviews.llvm.org/D135254