Better diagnostic for static override when methods are thiscall by default

Methods are thiscall by default in the MS ABI, and also in MinGW targetting GCC 4.7 or later.

This changes the diagnostic from the technically correct but hard to understand:

  virtual function 'foo' has different calling convention attributes ('void ()') than the function it overrides (which has calling convention 'void () __attribute__((thiscall))')

to the more intuitive and also correct:

  'static' member function 'foo' overrides a virtual function

We already have a test for this. Let's just run it in both ABI modes.

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