Make this const since it never changes (and should never change).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@182309 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/CGDebugInfo.h b/lib/CodeGen/CGDebugInfo.h
index 68fdbd9..f19f750 100644
--- a/lib/CodeGen/CGDebugInfo.h
+++ b/lib/CodeGen/CGDebugInfo.h
@@ -48,7 +48,7 @@
 /// the backend.
 class CGDebugInfo {
   CodeGenModule &CGM;
-  CodeGenOptions::DebugInfoKind DebugKind;
+  const CodeGenOptions::DebugInfoKind DebugKind;
   llvm::DIBuilder DBuilder;
   llvm::DICompileUnit TheCU;
   SourceLocation CurLoc, PrevLoc;