Convert a span_bug to a span_delayed_bug.

PR #121208 converted this from a `span_delayed_bug` to a `span_bug`
because nothing in the test suite caused execution to hit this path. But
now fuzzing has found a test case that does hit it. So this commit
converts it back to `span_delayed_bug` and adds the relevant test.

Fixes #126385.

compiler/rustc_borrowck/src/diagnostics/region_name.rs

@@ -628,9 +628,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
                     | GenericArgKind::Const(_),
                     _,
                 ) => {
-                    // This was previously a `span_delayed_bug` and could be
-                    // reached by the test for #82126, but no longer.
-                    self.dcx().span_bug(
+                    self.dcx().span_delayed_bug(
                         hir_arg.span(),
                         format!("unmatched arg and hir arg: found {kind:?} vs {hir_arg:?}"),
                     );

tests/ui/borrowck/unmatched-arg-and-hir-arg-issue-126385.rs

@@ -0,0 +1,14 @@
+// This test was triggering a `span_bug` crash, which was then fixed by
+// downgrading it to a `span_delayed_bug`.
+
+pub struct MyStruct<'field> {
+    field: &'field [u32],
+}
+
+impl MyStruct<'_> {
+    pub fn f(field: &[u32]) -> Self<u32> { //~ ERROR type arguments are not allowed on self type
+        Self { field }                     //~ ERROR lifetime may not live long enough
+    }
+}
+
+fn main() {}

tests/ui/borrowck/unmatched-arg-and-hir-arg-issue-126385.stderr

@@ -0,0 +1,34 @@
+error[E0109]: type arguments are not allowed on self type
+  --> $DIR/unmatched-arg-and-hir-arg-issue-126385.rs:9:37
+   |
+LL |     pub fn f(field: &[u32]) -> Self<u32> {
+   |                                ---- ^^^ type argument not allowed
+   |                                |
+   |                                not allowed on self type
+   |
+note: `Self` is of type `MyStruct<'_>`
+  --> $DIR/unmatched-arg-and-hir-arg-issue-126385.rs:4:12
+   |
+LL | pub struct MyStruct<'field> {
+   |            ^^^^^^^^ `Self` corresponds to this type
+...
+LL | impl MyStruct<'_> {
+   | ----------------- `Self` is on type `MyStruct` in this `impl`
+help: the `Self` type doesn't accept type parameters, use the concrete type's name `MyStruct` instead if you want to specify its type parameters
+   |
+LL |     pub fn f(field: &[u32]) -> MyStruct<u32> {
+   |                                ~~~~~~~~
+
+error: lifetime may not live long enough
+  --> $DIR/unmatched-arg-and-hir-arg-issue-126385.rs:10:9
+   |
+LL |     pub fn f(field: &[u32]) -> Self<u32> {
+   |                     -          --------- return type is MyStruct<'2>
+   |                     |
+   |                     let's call the lifetime of this reference `'1`
+LL |         Self { field }
+   |         ^^^^^^^^^^^^^^ associated function was supposed to return data with lifetime `'2` but it is returning data with lifetime `'1`
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0109`.