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:?}"),
                     );