support a way to check bitflags are exhaustive

Right now you just have to hope that whoever adds a new option to CodegenFnAttrFlags correctly updates all the places where it is relevant, e.g.

https://github.com/rust-lang/rust/blob/c4dc70ee0ad7f811fb32e5bed9cd6c7b37beed4e/compiler/rustc_middle/src/middle/codegen_fn_attrs.rs#L18-L50

cc @jdonszelmann https://github.com/rust-lang/rust/pull/146348

This is quite fragile and I certainly wouldn't have caught this omission in review.

A way to prevent such bugs is by exhaustively matching on all options in such places, e.g.
https://github.com/rust-lang/rust/blob/c4dc70ee0ad7f811fb32e5bed9cd6c7b37beed4e/compiler/rustc_middle/src/ty/context.rs#L632-L638

https://github.com/rust-lang/rust/blob/c4dc70ee0ad7f811fb32e5bed9cd6c7b37beed4e/compiler/rustc_next_trait_solver/src/solve/mod.rs#L72-L81

Imo we should extend our bitflags macro to also emit an enum of all options to enable us to exhaustively match on them in cases where this is likely useful.