[CIR] Add options to emit ClangIR and enable the ClangIR pipeline

Introduce just the option definitions and support for their existance at
a few different points in the frontend. This will be followed soon by
functionality that uses it.

Reviewers: bcardosolopes, jansvoboda11, AaronBallman, erichkeane, MaskRay

Reviewed By: erichkeane

Pull Request: https://github.com/llvm/llvm-project/pull/89030