Improve error recovery around colon.

Recognize additional cases, when '::' is mistyped as ':'.
This is a fix to RP18587 - colons have too much protection in member-declarations.

Differential Revision: http://reviews.llvm.org/D3653


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212957 91177308-0d34-0410-b5e6-96231b3b80d8