Resolve async fn signature even without body (e.g., in trait)

[compiler-errors]

Fixes #102138

This "bail if no body" behavior was introduced in #69539 to fix #69401, but that ICE does not reproduce any more. The error message changes a bit, but that's all, and I don't think it's a particularly diagnostic bad regression.

[rust-highfive]

r? @davidtwco

(rust-highfive has picked a reviewer for you, use r? to override)

[tmandry]

r=me after comments

[compiler-errors]

Ugh, actually this passes now and might not after the changes:

fn main() {}

trait Foo {
    fn fn_with_type_named_same_as_local_in_param(b: i32, b: i32);
}

Let me see if I can avoid this.

[compiler-errors]

Okay, fixed that bug by just doing the only part of resolve that we need for AFIT.

[davidtwco]

r? @tmandry

[tmandry]

@bors r+

[bors]

📌 Commit 271a0680197e08405c67dd0b4d4bac87ea9a28bc has been approved by tmandry

It is now in the queue for this repository.

[tmandry]

Ugh, actually this passes now and might not after the changes:

Actually, want to add a test for this? (and r=me)

Not sure it's the behavior I would want, but it's too late for that anyway :)

@bors r-

[compiler-errors]

@bors r=tmandry

[bors]

📌 Commit e87fcc0 has been approved by tmandry

It is now in the queue for this repository.