[MinGW] Reject explicit non-default visibility applied to dllexport/dllimport declaration

dllimport/dllexport is incompatible with protected/hidden visibilities.
(Arguably dllexport semantics is compatible with protected but let's reject the
combo for simplicity.)

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