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/Serialization/ASTWriter.cpp b/lib/Serialization/ASTWriter.cpp
index 67e03c2..8c14b5f 100644
--- a/lib/Serialization/ASTWriter.cpp
+++ b/lib/Serialization/ASTWriter.cpp
@@ -3238,10 +3238,12 @@
     Categories.push_back(0);
     
     // Add the categories.
-    for (ObjCCategoryDecl *Cat = Class->getCategoryList();
-         Cat; Cat = Cat->getNextClassCategory(), ++Size) {
-      assert(getDeclID(Cat) != 0 && "Bogus category");
-      AddDeclRef(Cat, Categories);
+    for (ObjCInterfaceDecl::known_categories_iterator
+           Cat = Class->known_categories_begin(),
+           CatEnd = Class->known_categories_end();
+         Cat != CatEnd; ++Cat, ++Size) {
+      assert(getDeclID(*Cat) != 0 && "Bogus category");
+      AddDeclRef(*Cat, Categories);
     }
     
     // Update the size.