[MLIR] Add support for defining Types in tblgen

Adds a TypeDef class to OpBase and backing generation code. Allows one
to define the Type, its parameters, and printer/parser methods in ODS.
Can generate the Type C++ class, accessors, storage class, per-parameter
custom allocators (for the storage constructor), and documentation.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D86904