Auto merge of #43426 - qnighy:intercrate-ambiguity-hints, r=nikomatsakis

Add hints when intercrate ambiguity causes overlap.

I'm going to tackle #23980.

# Examples

## Trait impl overlap caused by possible downstream impl

```rust
trait Foo<X> {}
trait Bar<X> {}
impl<X, T> Foo<X> for T where T: Bar<X> {}
impl<X> Foo<X> for i32 {}

fn main() {}
```

```
error[E0119]: conflicting implementations of trait `Foo<_>` for type `i32`:
 --> test1.rs:4:1
  |
3 | impl<X, T> Foo<X> for T where T: Bar<X> {}
  | ------------------------------------------ first implementation here
4 | impl<X> Foo<X> for i32 {}
  | ^^^^^^^^^^^^^^^^^^^^^^^^^ conflicting implementation for `i32`
  |
  = note: downstream crates may implement Bar

error: aborting due to previous error
```

## Trait impl overlap caused by possible upstream update

```rust
trait Foo {}
impl<T> Foo for T where T: ::std::fmt::Octal {}
impl Foo for () {}

fn main() {}
```

```
error[E0119]: conflicting implementations of trait `Foo` for type `()`:
 --> test2.rs:3:1
  |
2 | impl<T> Foo for T where T: ::std::fmt::Octal {}
  | ----------------------------------------------- first implementation here
3 | impl Foo for () {}
  | ^^^^^^^^^^^^^^^^^^ conflicting implementation for `()`
  |
  = note: upstream crates may add new impl for std::fmt::Octal in future versions

error: aborting due to previous error
```

## Inherent impl overlap caused by possible downstream impl

```rust
trait Bar<X> {}

struct A<T, X>(T, X);
impl<X, T> A<T, X> where T: Bar<X> { fn f(&self) {} }
impl<X> A<i32, X> { fn f(&self) {} }

fn main() {}
```

```
error[E0592]: duplicate definitions with name `f`
 --> test3.rs:4:38
  |
4 | impl<X, T> A<T, X> where T: Bar<X> { fn f(&self) {} }
  |                                      ^^^^^^^^^^^^^^ duplicate definitions for `f`
5 | impl<X> A<i32, X> { fn f(&self) {} }
  |                     -------------- other definition for `f`
  |
  = note: downstream crates may implement Bar

error: aborting due to previous error
```

## Inherent impl overlap caused by possible upstream update

```rust
struct A<T>(T);

impl<T> A<T> where T: ::std::fmt::Octal { fn f(&self) {} }
impl A<()> { fn f(&self) {} }

fn main() {}
```

```
error[E0592]: duplicate definitions with name `f`
 --> test4.rs:3:43
  |
3 | impl<T> A<T> where T: ::std::fmt::Octal { fn f(&self) {} }
  |                                           ^^^^^^^^^^^^^^ duplicate definitions for `f`
4 | impl A<()> { fn f(&self) {} }
  |              -------------- other definition for `f`
  |
  = note: upstream crates may add new impl for std::fmt::Octal in future versions

error: aborting due to previous error
```