Add note that this is for old bytecode files.

llvm-svn: 17991
diff --git a/llvm/lib/Bytecode/Reader/Reader.cpp b/llvm/lib/Bytecode/Reader/Reader.cpp
index bbf6952..e08656822d 100644
--- a/llvm/lib/Bytecode/Reader/Reader.cpp
+++ b/llvm/lib/Bytecode/Reader/Reader.cpp
@@ -1449,7 +1449,7 @@
     return Result;
   }
 
-  case Type::PointerTyID: {  // ConstantPointerRef value...
+  case Type::PointerTyID: {  // ConstantPointerRef value (backwards compat).
     const PointerType *PT = cast<PointerType>(Ty);
     unsigned Slot = read_vbr_uint();