[mlir][ods] Allow specifying return types of builders

This patch allows custom attribute and type builders to return
something other than the C++ type of the attribute or type.

This is useful for attributes or types that may perform extra work during
construction (e.g. canonicalization) that could result in a different
kind of attribute or type being returned.

Reviewed By: rriddle, lattner

Differential Revision: https://reviews.llvm.org/D129792