[clang] [llvm] [mlir] [TableGen] Change `DefInit::Def` to a const Record pointer (PR #110747)

[Matt Arsenault via cfe-commits]

================
@@ -1660,7 +1660,7 @@ class Record {
   // this record.
   SmallVector<SMLoc, 4> Locs;
   SmallVector<SMLoc, 0> ForwardDeclarationLocs;
-  SmallVector<SMRange, 0> ReferenceLocs;
+  mutable SmallVector<SMRange, 0> ReferenceLocs;
----------------
arsenm wrote:

You have the const_cast on the addition, so this is unnecessary? 

https://github.com/llvm/llvm-project/pull/110747