Improve error recovery around colon.

Parse of nested name spacifier is modified so that it properly recovers
if colon is mistyped as double colon in case statement.
This patch fixes PR15133.

Differential Revision: http://llvm-reviews.chandlerc.com/D2870


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206135 91177308-0d34-0410-b5e6-96231b3b80d8