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

"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.

Rename AccessibilityAttr to AccessControlAttr and
SetterAccessibilityAttr to SetterAccessAttr, then track down the last
few uses of "accessibility" that don't have to do with
NSAccessibility. (I left the SourceKit XPC API alone because that's
supposed to be more stable.)

lib/Sema/CSSimplify.cpp

@@ -3320,7 +3320,7 @@ retry_after_fail:
       includeInaccessibleMembers) {
     NameLookupOptions lookupOptions = defaultMemberLookupOptions;
     
-    // Ignore accessibility so we get candidates that might have been missed
+    // Ignore access control so we get candidates that might have been missed
     // before.
     lookupOptions |= NameLookupFlags::IgnoreAccessControl;
     // This is only used for diagnostics, so always use KnownPrivate.