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/Serialization/Serialization.cpp

@@ -1546,6 +1546,7 @@ static uint8_t getRawStableAccessibility(Accessibility access) {
   CASE(FilePrivate)
   CASE(Internal)
   CASE(Public)
+  CASE(Open)
 #undef CASE
   }
 }
@@ -3777,7 +3778,7 @@ static void writeDeclCommentTable(
         // we want to take the testability state into account
         // and emit documentation if and only if they are visible to clients
         // (which means public ordinarily, but public+internal when testing enabled).
-        if (VD->getEffectiveAccess() != Accessibility::Public)
+        if (VD->getEffectiveAccess() < Accessibility::Public)
           return true;
       }