- 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