[MLIR] ODS typedef gen fixes & improvements

- Fixes bug 48242 point 3 crash.
- Makes the improvments from points 1 & 2.

https://bugs.llvm.org/show_bug.cgi?id=48262

```
   def RTLValueType : Type<CPred<"isRTLValueType($_self)">, "Type"> {
     string cppType = "::mlir::Type";
   }
```
Works now, but merely by happenstance. Parameters expects a `TypeParameter` class def or a string representing a c++ type but doesn't enforce it.

Reviewed By: lattner

Differential Revision: https://reviews.llvm.org/D91939