[MinGW] Reject explicit hidden visibility applied to dllexport and...

[MinGW] Reject explicit hidden visibility applied to dllexport and hidden/protected applied to dllimport

Hidden visibility is incompatible with dllexport.
Hidden and protected visibilities are incompatible with dllimport.
(PlayStation uses dllexport protected.)

When an explicit visibility attribute applies on a dllexport/dllimport
declaration, report a Frontend error (Sema does not compute visibility).

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D133266