Pick one possible lifetime in case there are multiple choices
In case a lifetime variable is created, but doesn't have an obvious lifetime in the list of named lifetimes that it should be inferred to, just pick the first one for the diagnostic.
This happens e.g. in
```rust
fn foo<'a, 'b>(a: Struct<'a>, b: Struct<'b>) -> impl Trait<'a, 'b> {
if bar() { a } else { b }
}
```
where we get a lifetime variable that combines the lifetimes of `a` and `b` creating a lifetime that is the intersection of both. Right now the type system cannot express this and thus we get an error, but that error also can't express this.
I can also create an entirely new diagnostic that mentions all involved lifetimes, so it would actually mention `'a` and `'b` instead of just `'b`.
8c5114b4e8