Implement SE-0117.

One minor revision: this lifts the proposed restriction against
overriding a non-open method with an open one.  On reflection,
that was inconsistent with the existing rule permitting non-public
methods to be overridden with public ones.  The restriction on
subclassing a non-open class with an open class remains, and is
in fact consistent with the existing access rule.

lib/SIL/SILModule.cpp

@@ -274,6 +274,7 @@ static SILFunction::ClassVisibility_t getClassVisibility(SILDeclRef constant) {
     case Accessibility::Internal:
       return SILFunction::InternalClass;
     case Accessibility::Public:
+    case Accessibility::Open:
       return SILFunction::PublicClass;
   }
 }