Add a TODO and explain when we can get rid of the isMain field.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175932 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/IR/DIBuilder.cpp b/lib/IR/DIBuilder.cpp
index 16632a1..3b69119 100644
--- a/lib/IR/DIBuilder.cpp
+++ b/lib/IR/DIBuilder.cpp
@@ -98,7 +98,7 @@
     MDString::get(VMContext, Filename),
     MDString::get(VMContext, Directory),
     MDString::get(VMContext, Producer),
-    // Deprecate isMain field.
+    // isMain field can be removed when we remove the legacy debug info.
     ConstantInt::get(Type::getInt1Ty(VMContext), true), // isMain
     ConstantInt::get(Type::getInt1Ty(VMContext), isOptimized),
     MDString::get(VMContext, Flags),