modify leak-check to track only outgoing edges from placeholders

Also, update the affected tests. This seems strictly better but it is
actually more permissive than I initially intended. In particular it
accepts this

```
forall<'a, 'b> {
  exists<'intersection> {
    'a: 'intersection,
    'b: 'intersection,
  }
}
```

and I'm not sure I want to accept that. It implies that we have a
`'empty` in the new universe intoduced by the `forall`.

src/test/ui/issues/issue-57362-1.rs

@@ -17,7 +17,7 @@ impl<T> Trait for fn(&T) {
 
 fn f() {
     let a: fn(_) = |_: &u8| {};
-    a.f(); //~ ERROR no method named `f`
+    a.f(); //~ ERROR implementation of `Trait` is not general enough
 }
 
 fn main() {}

src/test/ui/issues/issue-57362-1.stderr

@@ -1,17 +1,16 @@
-error[E0599]: no method named `f` found for fn pointer `fn(&u8)` in the current scope
+error: implementation of `Trait` is not general enough
   --> $DIR/issue-57362-1.rs:20:7
    |
-LL |     a.f();
-   |       ^ method not found in `fn(&u8)`
+LL | / trait Trait {
+LL | |     fn f(self);
+LL | | }
+   | |_- trait `Trait` defined here
+...
+LL |       a.f();
+   |         ^ implementation of `Trait` is not general enough
    |
-   = note: `a` is a function, perhaps you wish to call it
-   = help: items from traits can only be used if the trait is implemented and in scope
-note: `Trait` defines an item `f`, perhaps you need to implement it
-  --> $DIR/issue-57362-1.rs:8:1
-   |
-LL | trait Trait {
-   | ^^^^^^^^^^^
+   = note: `Trait` would have to be implemented for the type `fn(&'0 u8)`, for some specific lifetime `'0`...
+   = note: ...but `Trait` is actually implemented for the type `for<'r> fn(&'r u8)`
 
 error: aborting due to previous error
 
-For more information about this error, try `rustc --explain E0599`.

src/test/ui/issues/issue-57362-2.rs

@@ -19,7 +19,9 @@ impl<'a> X for fn(&'a ()) {
 }
 
 fn g() {
-    let x = <fn (&())>::make_g(); //~ ERROR no function or associated item
+    let x = <fn (&())>::make_g();
+    //~^ ERROR implementation of `X` is not general enough
+    //~| ERROR implementation of `X` is not general enough
 }
 
 fn main() {}

src/test/ui/issues/issue-57362-2.stderr

@@ -1,16 +1,33 @@
-error[E0599]: no function or associated item named `make_g` found for fn pointer `for<'r> fn(&'r ())` in the current scope
-  --> $DIR/issue-57362-2.rs:22:25
+error: implementation of `X` is not general enough
+  --> $DIR/issue-57362-2.rs:22:13
    |
-LL |     let x = <fn (&())>::make_g();
-   |                         ^^^^^^ function or associated item not found in `for<'r> fn(&'r ())`
+LL | / trait X {
+LL | |     type G;
+LL | |     fn make_g() -> Self::G;
+LL | | }
+   | |_- trait `X` defined here
+...
+LL |       let x = <fn (&())>::make_g();
+   |               ^^^^^^^^^^^^^^^^^^ implementation of `X` is not general enough
    |
-   = help: items from traits can only be used if the trait is implemented and in scope
-note: `X` defines an item `make_g`, perhaps you need to implement it
-  --> $DIR/issue-57362-2.rs:8:1
-   |
-LL | trait X {
-   | ^^^^^^^
+   = note: `X` would have to be implemented for the type `for<'r> fn(&'r ())`
+   = note: ...but `X` is actually implemented for the type `fn(&'0 ())`, for some specific lifetime `'0`
 
-error: aborting due to previous error
+error: implementation of `X` is not general enough
+  --> $DIR/issue-57362-2.rs:22:13
+   |
+LL | / trait X {
+LL | |     type G;
+LL | |     fn make_g() -> Self::G;
+   | |     ----------------------- due to a where-clause on `X::make_g`...
+LL | | }
+   | |_- trait `X` defined here
+...
+LL |       let x = <fn (&())>::make_g();
+   |               ^^^^^^^^^^^^^^^^^^ doesn't satisfy where-clause
+   |
+   = note: ...`X` would have to be implemented for the type `for<'r> fn(&'r ())`
+   = note: ...but `X` is actually implemented for the type `fn(&'0 ())`, for some specific lifetime `'0`
+
+error: aborting due to 2 previous errors
 
-For more information about this error, try `rustc --explain E0599`.