Split PassMode::IndirectUnsized out of PassMode::Indirect

[bjorn3]

This prevents nonsensical combinations like unsized+on_stack and forces all users to explicitly handle the difference between sized and unsized arguments. In a couple of places unsized arguments were handled incorrectly if they would ever manage to reach that code.

[rustbot]

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

rustc_codegen_gcc is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_gcc instead.

cc @antoyo, @GuillaumeGomez

[rustbot]

r? @mati865

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[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[mati865]

Looks better to me, but would prefer a second look from somebody.

@bors reroll

[rust-bors]

Unknown command "reroll". Run @bors help or go to https://bors.rust-lang.org/help to see available commands.

[mati865]

Derp. @rustbot reroll