[clang] Rough first stab at addressing #85120 (PR #85147)

[via cfe-commits]

Sirraide wrote:

> > @dougsonos I may have some time to look into this since there are probably (as always) some annoying edge cases—particularly w/ templates. Would you be fine with me committing to this branch or would it be easier for you if I made a separate branch for that? Either is fine by me.
> 
> It might be simplest if you make your own branch, borrowing whatever from mine helps. Thanks.

Yeah, it turns out there are some general issues w/ `AttributedType`s, which means that a separate branch would make more sense; I’ll add you as a co-author if I end up taking code from this branch, because the part about checking for `AttributedType`s when rebuilding function types looks like it might be the way to go when it comes to not dropping the attribute, but we gotta fix the crashes first.

https://github.com/llvm/llvm-project/pull/85147