Forbid items with the same name from appearing in overlapping inherent impl blocks

For example, the following is now correctly illegal:

```rust
struct Foo;

impl Foo {
    fn id() {}
}

impl Foo {
    fn id() {}
}
```

"Overlapping" here is determined the same way it is for traits (and in fact shares the same code path): roughly, there must be some way of substituting any generic types to unify the impls, such that none of the `where` clauses are provably unsatisfiable under such a unification.

Along the way, this PR also introduces an `ImplHeader` abstraction (the first commit) that makes it easier to work with impls abstractly (without caring whether they are trait or inherent impl blocks); see the first commit.

Closes #22889
r? @nikomatsakis