Fix negative traits coherence checks compile error message for type aliases

[spastorino]

Follow up of this comment on #90104

This test https://github.com/rust-lang/rust/pull/90104/files#diff-2c291b05493bf924a05c167ef3923655ca244b6fd8bc2303ffdc2ed2132498cc is giving an ungreat error message.

#![feature(rustc_attrs)]
#![feature(trait_alias)]

trait A {}
trait B {}
trait AB = A + B;

impl A for u32 {}
impl B for u32 {}

trait C {}
#[rustc_strict_coherence]
impl<T: AB> C for T {}
#[rustc_strict_coherence]
impl C for u32 {}
//~^ ERROR
// FIXME it's giving an ungreat error but unsure if we care given that it's using an internal rustc
// attribute and an artificial code path for testing purposes

fn main() {}

fails with ...

error[E0283]: type annotations needed
  --> $DIR/coherence-overlap-trait-alias.rs:15:6
   |
LL | impl C for u32 {}
   |      ^ cannot infer type for type `u32`
   |
   = note: cannot satisfy `u32: C`
note: required by a bound in `C`
  --> $DIR/coherence-overlap-trait-alias.rs:11:1
   |
LL | trait C {}
   | ^^^^^^^ required by this bound in `C`

error: aborting due to previous error

Error should mention about conflicting implementations for u32 or something like that. More similar to what happens without using the rustc_strict_coherence attribute.

[spastorino]

As an effect of @estebank's PR #89427 this error is now a bit better but it's still not there compared with the no strict version.
Anyway, not a extremely important thing given that this can only be raised by using rustc_strict_coherence which is an internal rustc attribute.