Defer tail call ret ty equality to check_tail_calls

[compiler-errors]

Fixes #144892.

Currently the tail call signature check assumes that return types have been accounted for. However, this is not complete for several reasons.

Firstly, we were using subtyping instead of equality in the HIR typeck code:

rust/compiler/rustc_hir_typeck/src/expr.rs

Line 1096 in e1b9081

self.demand_suptype(expr.span, ret_ty, call_expr_ty);

We could fix this, but it doesn't really do much for us anyways since HIR typeck doesn't care about regions.

That means, secondly, we'd need to fix the terminator type check in MIR typeck to account for variances. This seems annoying.

All of this seems like a lot of work, and we already are manually checking argument equality. Let's just extend the check_tail_calls to account for mismatches in return types anyways.

r? @WaffleLapkin

[rustbot]

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