[Verifier] Reject dllexport with non-default visibility

[Verifier] Reject dllexport with non-default visibility

Add a visibility check for dllimport and dllexport. Note: dllimport with a
non-default visibility (implicit dso_local) is already rejected, but with a less
clear dso_local error.

The MC level visibility `MCSA_Exported` (D123951) is mapped from IR level
default visibility when dllexport is specified. The D123951 error is now very
difficult to trigger (needs to disable the IR verifier).

Reviewed By: mstorsjo

Differential Revision: https://reviews.llvm.org/D133267