[llvm] [TableGen] Support type aliases via new keyword deftype (PR #79570)

[Jon Roelofs via llvm-commits]

================
@@ -3665,6 +3671,36 @@ bool TGParser::ParseDefset() {
   return false;
 }
 
+/// ParseDeftype - Parse a defvar statement.
+///
+///   Deftype ::= DEFTYPE Id '=' Value ';'
----------------
jroelofs wrote:

```suggestion
///   Deftype ::= DEFTYPE Id '=' Type ';'
```

https://github.com/llvm/llvm-project/pull/79570