Always point at trait assoc item when generics don't match

Previously we only showed the trait's assoc item if the trait was local, because we were looking for a small span only for the generics, which we don't have for foreign traits. We now use `def_span` for the item, so we at least provide some context, even if its span is too wide.

```
error[E0195]: lifetime parameters or bounds on type `IntoIter` do not match the trait declaration
   --> tests/ui/lifetimes/missing-lifetime-in-assoc-type-4.rs:7:18
    |
7   |     type IntoIter<'a> = std::collections::btree_map::Values<'a, i32, T>;
    |                  ^^^^ lifetimes do not match type in trait
    |
   ::: /home/gh-estebank/rust/library/core/src/iter/traits/collect.rs:292:5
    |
292 |     type IntoIter: Iterator<Item = Self::Item>;
    |     ------------------------------------------ lifetimes in impl do not match this type in trait
```

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -1115,13 +1115,13 @@ fn check_region_bounds_on_impl_item<'tcx>(
             .expect("expected impl item to have generics or else we can't compare them")
             .span;
 
-        let mut generics_span = None;
+        let mut generics_span = tcx.def_span(trait_m.def_id);
         let mut bounds_span = vec![];
         let mut where_span = None;
         if let Some(trait_node) = tcx.hir().get_if_local(trait_m.def_id)
             && let Some(trait_generics) = trait_node.generics()
         {
-            generics_span = Some(trait_generics.span);
+            generics_span = trait_generics.span;
             // FIXME: we could potentially look at the impl's bounds to not point at bounds that
             // *are* present in the impl.
             for p in trait_generics.predicates {

compiler/rustc_hir_analysis/src/errors.rs

@@ -196,7 +196,7 @@ pub(crate) struct LifetimesOrBoundsMismatchOnTrait {
     #[label]
     pub span: Span,
     #[label(hir_analysis_generics_label)]
-    pub generics_span: Option<Span>,
+    pub generics_span: Span,
     #[label(hir_analysis_where_label)]
     pub where_span: Option<Span>,
     #[label(hir_analysis_bounds_label)]

tests/ui/lifetimes/missing-lifetime-in-assoc-type-4.stderr

@@ -11,6 +11,9 @@ error[E0195]: lifetime parameters or bounds on type `IntoIter` do not match the
    |
 LL |     type IntoIter<'a> = std::collections::btree_map::Values<'a, i32, T>;
    |                  ^^^^ lifetimes do not match type in trait
+  --> $SRC_DIR/core/src/iter/traits/collect.rs:LL:COL
+   |
+   = note: lifetimes in impl do not match this type in trait
 
 error: aborting due to 2 previous errors