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/SIL/SILDeclRef.cpp

@@ -207,7 +207,7 @@ SILLinkage SILDeclRef::getLinkage(ForDefinition_t forDefinition) const {
   }
   
   // Otherwise, we have external linkage.
-  switch (d->getAccessibility()) {
+  switch (d->getEffectiveAccess()) {
     case Accessibility::Private:
       return (forDefinition ? SILLinkage::Private : SILLinkage::PrivateExternal);