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/IRGen/IRGenModule.cpp

@@ -736,11 +736,11 @@ bool IRGenerator::canEmitWitnessTableLazily(SILWitnessTable *wt) {
     wt->getConformance()->getType()->getNominalOrBoundGenericNominal();
 
   switch (ConformingTy->getEffectiveAccess()) {
-    case Accessibility::Private:
-    case Accessibility::FilePrivate:
+    case AccessLevel::Private:
+    case AccessLevel::FilePrivate:
       return true;
 
-    case Accessibility::Internal:
+    case AccessLevel::Internal:
       return PrimaryIGM->getSILModule().isWholeModule();
 
     default: