Split getAccessibility() into getFormalAccess() and getEffectiveAccess().

Currently a no-op, but effective access for entities within the current
module will soon need to take testability into account. This declaration:

  internal func foo() {}

has a formal access of 'internal', but an effective access of 'public' if
we're in a testable mode.

Part of rdar://problem/17732115 (testability)

Swift SVN r26472

lib/IDE/ModuleInterfacePrinting.cpp

@@ -204,7 +204,7 @@ void swift::ide::printSubmoduleInterface(
     if (auto *VD = dyn_cast<ValueDecl>(D)) {
       if (Options.AccessibilityFilter > Accessibility::Private &&
           VD->hasAccessibility() &&
-          VD->getAccessibility() < Options.AccessibilityFilter)
+          VD->getFormalAccess() < Options.AccessibilityFilter)
         continue;
     }