Sema: Don't crash when solitary :: token appears before { in struct def

hasDeclaratorForAnonDecl, getDeclaratorForAnonDecl and
getTypedefNameForAnonDecl are expected to handle the case where
NamedDeclOrQualifier holds the wrong type or nothing at all.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224912 91177308-0d34-0410-b5e6-96231b3b80d8