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