Fold the six functions checking explicitly-defaulted special member functions

into one. These were all performing almost identical checks, with different bugs
in each of them.

This fixes PR12806 (we weren't setting the exception specification for an
explicitly-defaulted, non-user-provided default constructor) and enforces
8.4.2/2's rule that an in-class defaulted member must exactly match the implicit
parameter type.

llvm-svn: 156802