turn aligned-in-packed error into lint

[RalfJung]

Fixes #80926:
The aligned-in-packed check ignores the fact that generics exist, so it can trivially be bypassed. Acknowledge that by downgrading the hard error to a lint. The lint also only fires for repr(C) types, because that's the only case where that is a problem: the type may not actually match what the C compiler for the target does, if that C compiler is MSVC. This is tied up with rust-lang/rfcs#3845 and the general tension around repr(C) as a repr for predictable stable layout vs C compatibility. The error doesn't really help to make fixing the mess any easier though, so let's de-fang it.

Also fixes rust-lang/rfcs#3060; see that issue for a usecase that's made unnecessarily hard by the status quo.

Three years ago, the t-lang vibes seem to have been "yes let's downgrade this to a lint, that's kind of what it already is anyway".

Questions for t-lang:

• Are you still on-board with this?
• How should the lint be called? I went with aligned_fields_in_packed.
• When exactly should it fire? I currently require the outer packed type, the inner aligned type, and all the types in between to be repr(C). For Rust types I see no reason at all to forbid this combination, we can just define whatever we want for the layout there and IMO the current behavior makes a lot of sense.
• What should the default lint level be? I went with "deny" since it was a hard error after all.

Cc @rust-lang/opsem

[rustbot]

r? @mu001999

rustbot has assigned @mu001999.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 75 candidates
• Random selection from 21 candidates