Stabilize the following methods of `Result` as const:
 - `is_ok`
 - `is_err`
 - `as_ref`

Possible because of stabilization of rust-lang#49146 (Allow if and match in constants).

Part of rust-lang#76268

Update the test `redundant_pattern_matching`: check if `is_ok` and `is_err` are suggested within const contexts.
Also removes the `redundant_pattern_matching_const_result` test, as it is no longer needed.

`is_ok` and `is_err` are stabilized as const and can thus always be suggested.

Co-authored-by: Ralf Jung <post@ralfj.de>