Add a quick TODO.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@151217 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/CGDebugInfo.cpp b/lib/CodeGen/CGDebugInfo.cpp
index 4fad9f6..d1dbe41 100644
--- a/lib/CodeGen/CGDebugInfo.cpp
+++ b/lib/CodeGen/CGDebugInfo.cpp
@@ -345,6 +345,7 @@
     //  Class isa;
     // } *id;
 
+    // TODO: Cache these two types to avoid duplicates.
     llvm::DIType OCTy =
       DBuilder.createForwardDecl(llvm::dwarf::DW_TAG_structure_type,
                                  "objc_class", getOrCreateMainFile(),