[NLL] "'static requirement introduced here" note missing

[jackh726]

In the various revisions of src/test/ui/associated-types/cache/project-fn-ret-invariant.rs, we go from

error[E0759]: `x` has lifetime `'a` but it needs to satisfy a `'static` lifetime requirement
  --> $DIR/project-fn-ret-invariant.rs:55:9
   |
LL | fn baz<'a, 'b>(x: Type<'a>) -> Type<'static> {
   |                   -------- this data with lifetime `'a`...
...
LL |     bar(foo, x)
   |         ^^^  - ...is used and required to live as long as `'static` here
   |
note: `'static` lifetime requirement introduced by the return type
  --> $DIR/project-fn-ret-invariant.rs:51:37
   |
LL | fn baz<'a, 'b>(x: Type<'a>) -> Type<'static> {
   |                                     ^^^^^^^ `'static` requirement introduced here
...
LL |     bar(foo, x)
   |     ----------- because of this returned expression

to

error: lifetime may not live long enough
  --> $DIR/project-fn-ret-invariant-nll.rs:56:5
   |
LL | fn baz<'a, 'b>(x: Type<'a>) -> Type<'static> {
   |        -- lifetime `'a` defined here
...
LL |     bar(foo, x)
   |     ^^^^^^^^^^^ returning this value requires that `'a` must outlive `'static`
   |
   = note: requirement occurs because of the type `Type<'_>`, which makes the generic argument `'_` invariant
   = note: the struct `Type<'a>` is invariant over the parameter `'a`
   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance

We lose the note, particularly the bit pointing to the return type (it's sort of in the note, but not exactly)

[compiler-errors]

This regression is related to #97267 (kinda, not really, but bear with me) -- all we have is the two regions' spans and a ConstraintCategory that tells us to print "returning this value" before "requires that [...]".

We could add more rich information into ConstraintCategory::Return, perhaps? Otherwise we really just don't have enough info funnelled down to where we're actually emitting errors to do much more than just print that it's a return value causing this outlives constraint.