Reject varargs '...' in function prototype if there are more parameters after

it. Diagnose with recovery if it appears after a function parameter that was
obviously supposed to be a parameter pack. Otherwise, warn if it immediately
follows a function parameter pack, because the user most likely didn't intend
to write a parameter pack followed by a C-style varargs ellipsis.

This warning can be syntactically disabled by using ", ..." instead of "...".

llvm-svn: 215408