Rework the traversal of Objective-C categories and extensions to
consider (sub)module visibility.

The bulk of this change replaces myriad hand-rolled loops over the
linked list of Objective-C categories/extensions attached to an
interface declaration with loops using one of the four new category
iterator kinds:

  visible_categories_iterator: Iterates over all visible categories
  and extensions, hiding any that have their "hidden" bit set. This is
  by far the most commonly used iterator.

  known_categories_iterator: Iterates over all categories and
  extensions, ignoring the "hidden" bit. This tends to be used for
  redeclaration-like traversals.

  visible_extensions_iterator: Iterates over all visible extensions,
  hiding any that have their "hidden" bit set.

  known_extensions_iterator: Iterates over all extensions, whether
  they are visible to normal name lookup or not.

The effect of this change is that any uses of the visible_ iterators
will respect module-import visibility. See the new tests for examples.

Note that the old accessors for categories and extensions are gone;
there are *Raw() forms for some of them, for those (few) areas of the
compiler that have to manipulate the linked list of categories
directly. This is generally discouraged.

Part two of <rdar://problem/10634711>.
 



git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172665 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Rewrite/Frontend/RewriteObjC.cpp b/lib/Rewrite/Frontend/RewriteObjC.cpp
index 23719cb..18b7575 100644
--- a/lib/Rewrite/Frontend/RewriteObjC.cpp
+++ b/lib/Rewrite/Frontend/RewriteObjC.cpp
@@ -5744,11 +5744,8 @@
                                               std::string &Result) {
   ObjCInterfaceDecl *ClassDecl = IDecl->getClassInterface();
   // Find category declaration for this implementation.
-  ObjCCategoryDecl *CDecl;
-  for (CDecl = ClassDecl->getCategoryList(); CDecl;
-       CDecl = CDecl->getNextClassCategory())
-    if (CDecl->getIdentifier() == IDecl->getIdentifier())
-      break;
+  ObjCCategoryDecl *CDecl
+    = ClassDecl->FindCategoryDeclaration(IDecl->getIdentifier());
   
   std::string FullCategoryName = ClassDecl->getNameAsString();
   FullCategoryName += '_';