C++ core issue 1344, PR10618: promote "addition of default argument makes this

a special member" diagnostic from warning to error, and fix the cases where it
produced diagnostics with incorrect wording.

We don't support this as an extension, and we ban it even in C++98 mode. This
breaks too much (for instance, the ABI-specified calling convention for a type
can change if it acquires a copy constructor through the addition of a default
argument).

llvm-svn: 168769