Address some comments on the name lookup/DeclContext patch from Chris

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@60897 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/AST/ASTContext.cpp b/lib/AST/ASTContext.cpp
index e62920f..1647e75 100644
--- a/lib/AST/ASTContext.cpp
+++ b/lib/AST/ASTContext.cpp
@@ -539,6 +539,7 @@
   ASTRecordLayout *NewEntry = new ASTRecordLayout();
   Entry = NewEntry;
 
+  // FIXME: Avoid linear walk through the fields, if possible.
   NewEntry->InitializeLayout(std::distance(D->field_begin(), D->field_end()));
   bool IsUnion = D->isUnion();