[llvm] [TableGen] Add let append/prepend syntax for field concatenation (PR #182382)

[Rahul Joshi via llvm-commits]

================
@@ -3637,8 +3689,15 @@ bool TGParser::ParseBodyItem(Record *CurRec) {
     return false;
   }
 
-  // LET ID OptionalRangeList '=' Value ';'
-  if (Lex.Lex() != tgtok::Id)
+  // LET [append|prepend] ID OptionalBitList '=' Value ';'
+  Lex.Lex(); // eat 'let'.
+
+  if (Lex.getCode() != tgtok::Id)
----------------
jurahul wrote:

what happens if we have existing fields with names append or prepend?

```
class X {
  int append = 0;
}

class Y : X {
  let append = 1;
}
```

This I think will parse currently and fail with the new code? Should we interpret the `append` and `prepend` as LetModes only when they are followed with an identifier?

https://github.com/llvm/llvm-project/pull/182382