Improve the diagnostic when a comma ends up at the end of a declarator group

Improve the diagnostic when a comma ends up at the end of a declarator group
instead of a semicolon (as sometimes happens during refactorings). When such a
comma is seen at the end of a line, and is followed by something which can't
possibly be a declarator (or even something which might be a plausible typo for
a declarator), suggest that a semicolon was intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@142544 91177308-0d34-0410-b5e6-96231b3b80d8