Fix invalid special casing of the unreachable! macro

[Urgau]

This pull-request fix an invalid special casing of the unreachable! macro in the same way the panic! macro was solved, by adding two new internal only macros unreachable_2015 and unreachable_2021 edition dependent and turn unreachable! into a built-in macro that do dispatching. This logic is stolen from the panic! macro.

This pull-request also adds an internal feature format_args_capture_non_literal that allows capturing arguments from formatted string that expanded from macros. The original RFC #2795 mentioned this as a future possibility. This feature is required because of concatenation that needs to be done inside the macro:

$crate::concat!("internal error: entered unreachable code: ", $fmt)

In summary the new behavior for the unreachable! macro with this pr is:

Edition 2021:

let x = 5;
unreachable!("x is {x}");

internal error: entered unreachable code: x is 5

Edition <= 2018:

let x = 5;
unreachable!("x is {x}");

internal error: entered unreachable code: x is {x}

Also note that the change in this PR are insta-stable and breaking changes but this a considered as being a bug.
If someone could start a perf run and then a crater run this would be appreciated.

Fixes #92137