Infer all inference variables via InferCx

The previous algorithm was correct for the example given in its
documentation, but when the TAIT was declared as a free item
instead of an associated item, the generic parameters were the
wrong ones.

src/test/ui/type-alias-impl-trait/issue-60371.rs

@@ -9,7 +9,7 @@ trait Bug {
 impl Bug for &() {
     type Item = impl Bug; //~ ERROR `impl Trait` in type aliases is unstable
     //~^ ERROR the trait bound `(): Bug` is not satisfied
-    //~^^ ERROR could not find defining uses
+    //~^^ ERROR the trait bound `(): Bug` is not satisfied
 
     const FUN: fn() -> Self::Item = || ();
     //~^ ERROR type alias impl trait is not permitted here

src/test/ui/type-alias-impl-trait/issue-60371.stderr

@@ -25,11 +25,14 @@ LL |     type Item = impl Bug;
    = help: the following implementations were found:
              <&() as Bug>
 
-error: could not find defining uses
+error[E0277]: the trait bound `(): Bug` is not satisfied
   --> $DIR/issue-60371.rs:10:17
    |
 LL |     type Item = impl Bug;
-   |                 ^^^^^^^^
+   |                 ^^^^^^^^ the trait `Bug` is not implemented for `()`
+   |
+   = help: the following implementations were found:
+             <&() as Bug>
 
 error: aborting due to 4 previous errors