Fix diagnostic for static methods referencing fields from using decls

Previously we thought the instance member was a function, not a field,
and we'd say something silly like:
  t.cpp:4:27: error: call to non-static member function without an object argument
    static int f() { return n; }
                            ^

Noticed in PR21923.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@224480 91177308-0d34-0410-b5e6-96231b3b80d8