[SILOpt] Use getEffectiveAccess instead of getFormalAccess.

Since this code has an isAvailableExternally check, this probably makes
no difference, but it's still more correct: SIL-level code should
always use a declaration's effective access.

Author: jrose-apple

lib/SILOptimizer/IPO/GlobalOpt.cpp

@@ -740,13 +740,14 @@ static bool canBeChangedExternally(SILGlobalVariable *SILG) {
   // Use access specifiers from the declarations,
   // if possible.
   if (auto *Decl = SILG->getDecl()) {
-    auto Access = Decl->getFormalAccess();
-    if (Access == Accessibility::Private)
+    switch (Decl->getEffectiveAccess()) {
+    case Accessibility::Private:
       return false;
-    if (Access == Accessibility::Internal &&
-        SILG->getModule().isWholeModule())
-      return false;
-    return true;
+    case Accessibility::Internal:
+      return !SILG->getModule().isWholeModule();
+    case Accessibility::Public:
+      return true;
+    }
   }
 
   if (SILG->getLinkage() == SILLinkage::Private)