librustc: Change the syntax of subslice matching to use postfix `..`

instead of prefix `..`.

This breaks code that looked like:

    match foo {
        [ first, ..middle, last ] => { ... }
    }

Change this code to:

    match foo {
        [ first, middle.., last ] => { ... }
    }

RFC #55.

Closes #16967.

[breaking-change]

r? @brson
This could be mod_spans.iter().skip(1) and then neither the above span_note nor the if i > 1 branch would be necessary.