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/Index/IndexSymbol.cpp

@@ -70,7 +70,7 @@ static bool isUnitTest(const ValueDecl *D) {
   if (FD->getParameterList(1)->size() != 0)
     return false;
 
-  // 5. ...is of at least 'internal' accessibility (unless we can use
+  // 5. ...is of at least 'internal' access (unless we can use
   //    Objective-C reflection)...
   if (!D->getASTContext().LangOpts.EnableObjCInterop &&
       (D->getFormalAccess() < AccessLevel::Internal ||