Don't pick `T: FnPtr` nested goals as the leaf goal in diagnostics for new solver
r? `@lcnr`
See `tests/ui/traits/next-solver/diagnostics/dont-pick-fnptr-bound-as-leaf.rs` for a minimized example of what code this affects the diagnostics off. The output of running nightly `-Znext-solver` on that test is the following:
```
error[E0277]: the trait bound `Foo: Trait` is not satisfied
--> src/lib.rs:14:20
|
14 | requires_trait(Foo);
| -------------- ^^^ the trait `FnPtr` is not implemented for `Foo`
| |
| required by a bound introduced by this call
|
note: required for `Foo` to implement `Trait`
--> src/lib.rs:7:16
|
7 | impl<T: FnPtr> Trait for T {}
| ----- ^^^^^ ^
| |
| unsatisfied trait bound introduced here
note: required by a bound in `requires_trait`
--> src/lib.rs:11:22
|
11 | fn requires_trait<T: Trait>(_: T) {}
| ^^^^^ required by this bound in `requires_trait`
```
Part of rust-lang/trait-system-refactor-initiative#148
318466aec0