[mlir][ods] Allow parameters to be default constructible

[Mogball]

Attribute and type parameters in custom parsers are populated with FailureOr<T> & because the parser does not assume they are default constructible. This is cumbersome and ugly. Parameters in TableGen should be markable as defaultConstructible so that we can pass a T & instead.

[llvmbot]

@llvm/issue-subscribers-mlir-core

[zero9178]

I think instead of requiring to mark it within TableGen, ODS should just generate code making use of https://en.cppreference.com/w/cpp/types/is_default_constructible if possible to let the C++ compiler be the source of truth.
With the negative effect ofc of the generated code being less readable and more complex.

[zero9178]

Candidate patch: https://reviews.llvm.org/D142301