[TableGen] Support type aliases via new keyword deftype

We can use `deftype` (not using `typedef` here to be consistent
with `def`, `defm`, `defset`, `defvar`, etc) to define type aliases.

Currently, only primitive types and type aliases are supported to be
the source type and `deftype` statements can only appear at the top
level.

Reviewers: fpetrogalli, Artem-B, nhaehnle, jroelofs

Reviewed By: jroelofs, nhaehnle, Artem-B

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