Parse: Consume tokens more carefully in CheckForLParenAfterColonColon

We would consume the lparen even if it wasn't followed by an identifier
or a star-identifier pair.

This fixes PR21815.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224403 91177308-0d34-0410-b5e6-96231b3b80d8