Allow breaking after "::" if absolutely necessary.

Otherwise, really long nested name specifiers can easily lead to a
violation of the column limit.

Not sure about the rules for indentation in those cases, so input is
appreciated (see tests.).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@173438 91177308-0d34-0410-b5e6-96231b3b80d8