diagnostics: account for self type when looking for source of unsolved type variable
Fixes#109905.
When searching for the source of an unsolved infer var inside of a list of generic args, we look through the `tcx.generics_of(…).own_substs(…)` which *skips* the self type if present. However, the computed `argument_index` is later[^1] used to index into `tcx.generics_of(…).params` which may still contain the self type. In such case, we are off by one when indexing into the parameters.
From now on, we account for this immediately after calling `own_substs` which keeps things local.
This also fixes the wrong output in the preexisting UI test `inference/need_type_info/concrete-impl.rs` which was overlooked. It used to claim that the *type of type parameter `Self`* couldn't be inferred in `<Struct as Ambiguous<_>>::method()` which of course isn't true: `Self` equals `Struct` here, `A` couldn't be inferred.
`@rustbot` label A-diagnostics
[^1]: f98a271814/compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs (L471)
