Do not ICE in type-alias-impl-trait with save-analysis

src/test/ui/save-analysis/issue-68621.rs

@@ -0,0 +1,17 @@
+// compile-flags: -Zsave-analysis
+
+#![feature(type_alias_impl_trait)]
+
+trait Trait {}
+
+trait Service {
+    type Future: Trait;
+}
+
+struct Struct;
+
+impl Service for Struct {
+    type Future = impl Trait; //~ ERROR: could not find defining uses
+}
+
+fn main() {}

src/test/ui/save-analysis/issue-68621.stderr

@@ -0,0 +1,8 @@
+error: could not find defining uses
+  --> $DIR/issue-68621.rs:14:5
+   |
+LL |     type Future = impl Trait;
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+