[modules] Update visibility for merged ObjCProtocolDecl definitions.

Merge definition visibility the same way we do for other decls. Without
the fix the added test emits `-Wobjc-method-access` as it cannot find a
visible protocol. Make this warning `-Werror` so the test would fail
when protocol visibility regresses.

rdar://83600696

Differential Revision: https://reviews.llvm.org/D111860