[PATCH] Better diagnostic for static function overriding thiscall virtual method

[Richard Smith]

  Maybe also remove the old (and now unreachable) check from `CheckFunctionDeclaration`? This slightly degrades our diagnostic in this case: we get (one error + one note) per overridden function, rather than one error + (one note per overridden function), but I can live with that.

http://llvm-reviews.chandlerc.com/D2375