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/SILPasses/Devirtualizer.cpp

@@ -254,7 +254,7 @@ static bool isDefaultCaseKnown(ClassHierarchyAnalysis *CHA,
     return false;
 
   // Only consider 'private' members, unless we are in whole-module compilation.
-  switch (CD->getAccessibility()) {
+  switch (CD->getEffectiveAccess()) {
   case Accessibility::Public:
     return false;
   case Accessibility::Internal: