Excise "Accessibility" from the compiler (2/3)

"Accessibility" has a different meaning for app developers, so we've
already deliberately excised it from our diagnostics in favor of terms
like "access control" and "access level". Do the same in the compiler
now that we aren't constantly pulling things into the release branch.

This commit changes the 'Accessibility' enum to be named 'AccessLevel'.

lib/AST/Attr.cpp

@@ -591,15 +591,15 @@ StringRef DeclAttribute::getAttrName() const {
   case DAK_Accessibility:
   case DAK_SetterAccessibility:
     switch (cast<AbstractAccessibilityAttr>(this)->getAccess()) {
-    case Accessibility::Private:
+    case AccessLevel::Private:
       return "private";
-    case Accessibility::FilePrivate:
+    case AccessLevel::FilePrivate:
       return "fileprivate";
-    case Accessibility::Internal:
+    case AccessLevel::Internal:
       return "internal";
-    case Accessibility::Public:
+    case AccessLevel::Public:
       return "public";
-    case Accessibility::Open:
+    case AccessLevel::Open:
       return "open";
     }
     llvm_unreachable("bad accessibility kind");