-ms-extensions: Implement __super scope specifier (PR13236).

We build a NestedNameSpecifier that records the CXXRecordDecl in which
__super appeared. Name lookup is performed in all base classes of the
recorded CXXRecordDecl. Use of __super is allowed only inside class and
member function scope.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218484 91177308-0d34-0410-b5e6-96231b3b80d8