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