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
llvm-svn: 175480
John McCall