Move has_self field to hir::AssocKind::Fn.

`hir::AssocItem` currently has a boolean `fn_has_self_parameter` field,
which is misplaced, because it's only relevant for associated fns, not
for associated consts or types. This commit moves it (and renames it) to
the `AssocKind::Fn` variant, where it belongs.

This requires introducing a new C-style enum, `AssocTag`, which is like
`AssocKind` but without the fields. This is because `AssocKind` values
are passed to various functions like `find_by_ident_and_kind` to
indicate what kind of associated item should be searched for, and having
to specify `has_self` isn't relevant there.

New methods:
- Predicates `AssocItem::is_fn` and `AssocItem::is_method`.
- `AssocItem::as_tag` which converts `AssocItem::kind` to `AssocTag`.

Removed `find_by_name_and_kinds`, which is unused.

`AssocItem::descr` can now distinguish between methods and associated
functions, which slightly improves some error messages.

tests/ui/generic-associated-types/impl_bounds.stderr

@@ -57,14 +57,14 @@ note: required for `Fooy<T>` to implement `Copy`
    |
 LL | #[derive(Copy, Clone)]
    |          ^^^^ unsatisfied trait bound introduced in this `derive` macro
-note: the requirement `Fooy<T>: Copy` appears on the `impl`'s method `d` but not on the corresponding trait's method
+note: the requirement `Fooy<T>: Copy` appears on the `impl`'s associated function `d` but not on the corresponding trait's associated function
   --> $DIR/impl_bounds.rs:7:8
    |
 LL | trait Foo {
    |       --- in this trait
 ...
 LL |     fn d() where Self: Clone;
-   |        ^ this trait's method doesn't have the requirement `Fooy<T>: Copy`
+   |        ^ this trait's associated function doesn't have the requirement `Fooy<T>: Copy`
 help: consider restricting type parameter `T` with trait `Copy`
    |
 LL | impl<T: std::marker::Copy> Foo for Fooy<T> {

tests/ui/traits/next-solver/normalize/normalize-param-env-2.stderr

@@ -4,14 +4,14 @@ error[E0275]: overflow evaluating the requirement `<() as A<T>>::Assoc: A<T>`
 LL |         Self::Assoc: A<T>,
    |                      ^^^^
    |
-note: the requirement `<() as A<T>>::Assoc: A<T>` appears on the `impl`'s method `f` but not on the corresponding trait's method
+note: the requirement `<() as A<T>>::Assoc: A<T>` appears on the `impl`'s associated function `f` but not on the corresponding trait's associated function
   --> $DIR/normalize-param-env-2.rs:12:8
    |
 LL | trait A<T> {
    |       - in this trait
 ...
 LL |     fn f()
-   |        ^ this trait's method doesn't have the requirement `<() as A<T>>::Assoc: A<T>`
+   |        ^ this trait's associated function doesn't have the requirement `<() as A<T>>::Assoc: A<T>`
 
 error[E0275]: overflow evaluating the requirement `<() as A<T>>::Assoc: A<T>`
   --> $DIR/normalize-param-env-2.rs:24:22