[Feature request] non_exhaustive structs

From a C API point of view, adding a field to a struct is often not seen as a breaking change, for fields that are not mentioned when giving a struct by value are zeroed. The Rust equivalent would be to make all structs non_exhaustive (which, unless the type is not supposed to be constructed from Rust, only makes sense if Default is derived on it).

Therefore, I think it would make sense to enable having non_exhaustive on structs, so that more C non-breaking changes can be translated to Rust non-breaking changes. A -sys crate using this would merely need to make the decision of whether either

1. the underlying C library only adds fields at the end (or in alignment-guaranteed gaps), or
2. the -sys crate makes the concession that the #[repr(C)] merely guarantees that it is aligned with the linked C code, but no concrete layout guarantee.

Is this something that you agree would make sense to have in bindgen (say, as an optional "make all (or selected) structs non_exhaustive")? Is there anything that a PR implementing this could do to make 2. go over better? (I don't think we can make the repr property non-pub, but maybe there are tricks I'm missing.)

The issue names no files or tests. Start by locating bindgen's struct-generation and configuration paths, then inspect how repr(C), Default, and other generated attributes are handled. Done means an optional setting can apply non_exhaustive to all or selected structs, with the layout and C-compatibility trade-offs covered by tests.