Arbitrary self types: diagnostics for mismatched self types

[adetaylor]

Add extra diagnostics for cases where self types don't match.

Most of this is new tests. These are slightly duplicative of the existing arbitrary-self-from-method-substs.rs test, but test more permutations so it seems worth adding to the coverage as we explore improving the diagnostics here.

Improved diagnostics were suggested in commit 05c5caa

This is a part of the arbitrary self types v2 project, rust-lang/rfcs#3519
and specifically the sub-issue exploring questions around generics,
#129147

Tracking:

• Tracking issue for RFC 3519: arbitrary_self_types #44874

[rustbot]

r? @jieyouxu

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[adetaylor]

@rustbot label +S-waiting-on-author -S-waiting-on-review

[adetaylor]

r? @wesleywiser

(but not ready for review yet)