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

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 "...".

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215408 91177308-0d34-0410-b5e6-96231b3b80d8