Add optional messages to the unreachable macro.

[michaelsproul]

Closes #18842.

[michaelsproul]

As brought up by @bstrie in #18842, having the option to print a message when unreachable code runs can be useful, particularly when tweaking existing code.

You can now do:

unreachable!();
unreachable!("oh no!");
unreachable!("wow, formatting: {}", 7);

Corresponding output:

internal error: entered unreachable code
internal error: entered unreachable code: oh no!
internal error: entered unreachable code: wow, formatting: 7

Calling the macro without an argument preserves the current behaviour.