Add `ConstArgKind::Path` and make `ConstArg` its own HIR node

This is a very large commit since a lot needs to be changed in order to
make the tests pass. The salient changes are:

- `ConstArgKind` gets a new `Path` variant, and all const params are now
  represented using it. Non-param paths still use `ConstArgKind::Anon`
  to prevent this change from getting too large, but they will soon use
  the `Path` variant too.

- `ConstArg` gets a distinct `hir_id` field and its own variant in
  `hir::Node`. This affected many parts of the compiler that expected
  the parent of an `AnonConst` to be the containing context (e.g., an
  array repeat expression). They have been changed to check the
  "grandparent" where necessary.

- Some `ast::AnonConst`s now have their `DefId`s created in
  rustc_ast_lowering rather than `DefCollector`. This is because in some
  cases they will end up becoming a `ConstArgKind::Path` instead, which
  has no `DefId`. We have to solve this in a hacky way where we guess
  whether the `AnonConst` could end up as a path const since we can't
  know for sure until after name resolution (`N` could refer to a free
  const or a nullary struct). If it has no chance as being a const
  param, then we create a `DefId` in `DefCollector` -- otherwise we
  decide during ast_lowering. This will have to be updated once all path
  consts use `ConstArgKind::Path`.

- We explicitly use `ConstArgHasType` for array lengths, rather than
  implicitly relying on anon const type feeding -- this is due to the
  addition of `ConstArgKind::Path`.

- Some tests have their outputs changed, but the changes are for the
  most part minor (including removing duplicate or almost-duplicate
  errors). One test now ICEs, but it is for an incomplete, unstable
  feature and is now tracked at #127009.

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