Use the actual class visibility for the ObjC EHTYPE global,
not the global visibility mode.

Noticed by inspection.
Add support for -fvisibility-ms-compat.

We treat this as an alternative to -fvisibility=<?>
which changes the default value visibility to "hidden"
and the default type visibility to "default".

Expose a -cc1 option for changing the default type
visibility, repurposing -fvisibility as the default
value visibility option (also setting type visibility
from it in the absence of a specific option).

rdar://13079314