Parse: Don't crash when namespace is in GNU statement expr

Parse: Don't crash when namespace is in GNU statement expr

Parser::ParseNamespace can get a little confused when it found itself
inside a compound statement inside of a non-static data member
initializer.

Try to determine that the statement expression's scope makes sense
before trying to parse it's contents.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@225514 91177308-0d34-0410-b5e6-96231b3b80d8