Make getDiagnosticsInGroup helper method a static function in the cpp file and move the WarningOption struct into an anonymous namespace instead of clang namespace since it no longer needs to be forward declared in the header.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189569 91177308-0d34-0410-b5e6-96231b3b80d8