[clang] [Clang] Consider preferred_type in bitfield warnings (#116760) (PR #116785)

[Vlad Serebrennikov via cfe-commits]

Endilll wrote:

> and I thought making just preferred_type warnings early would be especially insane :D

I don't necessarily agree. `preferred_type` is a relatively new thing, so we can try doing something new here.

> [edit: I just realized even that might need to be an option because I can imagine someone, somewhere, depending on -Werror and not hitting this by luck]

I don't believe we need to be concerned about forward compatibility for `-Werror` users. They should know they signed themselves up for pain, and we're not going to compromise our QoI because of that.

In any case, this PR is step in the right direction, so I don't want to block its progress.

https://github.com/llvm/llvm-project/pull/116785