Improve diagnostics for missing associated types

[alexreg]

When the user fails to specify associated types for a trait object, the span used for the label associated type `Output` must be specified is the entire trait object, when it should ideally just be the trait (bound) from which the associated type is missing.

For example, we currently get:

3 | type Foo = Add + Sub;
  |            ^^^^^^^^^ associated type `Output` must be specified

when we really want something like:

3 | type Foo = Add + Sub;
  |            |     ^^^ associated type `Output` must be specified
  |            ^^^^      associated type `Output` must be specified

Note that PR #59445 will ensure that there at least are two labels in the above example, which is more correct, although the spans of both labels will still be the entire trait object.

See also #59445 (comment).

CC @estebank @eddyb @Centril

[eddyb]

cc @varkor

[estebank]

Current output:

error[E0191]: the value of the associated types `Output` (from the trait `std::ops::Add`), `Output` (from the trait `std::ops::Sub`) must be specified
 --> src/lib.rs:1:12
  |
1 | type Foo = std::ops::Add + std::ops::Sub;
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |            |
  |            associated type `Output` must be specified
  |            associated type `Output` must be specified

It mentions all of the types that need the associated type set, but the span is still the same.

[estebank]

#67268 touches this part of the codebase. It doesn't address it, but it should be possible to build on it to do so. With that PR, the output is

error[E0191]: the value of the associated types `Output` (from trait `std::ops::Add`), `Output` (from trait `std::ops::Sub`) must be specified
 --> file12.rs:1:12
  |
1 | type Foo = std::ops::Add + std::ops::Sub;
  |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ associated types `Output`, `Output` must be specified