Fix crash with TAIT in the call codegen code

The new logic is closer to what cg_llvm does.

Fixes #1240

example/issue-72793.rs

@@ -0,0 +1,24 @@
+// Adapted from rustc ui test suite (ui/type-alias-impl-trait/issue-72793.rs)
+
+#![feature(type_alias_impl_trait)]
+
+trait T { type Item; }
+
+type Alias<'a> = impl T<Item = &'a ()>;
+
+struct S;
+impl<'a> T for &'a S {
+    type Item = &'a ();
+}
+
+fn filter_positive<'a>() -> Alias<'a> {
+    &S
+}
+
+fn with_positive(fun: impl Fn(Alias<'_>)) {
+    fun(filter_positive());
+}
+
+fn main() {
+    with_positive(|_| ());
+}