librustc: Forbid `..` in range patterns.

This breaks code that looks like:

    match foo {
        1..3 => { ... }
    }

Instead, write:

    match foo {
        1...3 => { ... }
    }

Closes #17295.

[breaking-change]
auto merge of #17584 : pcwalton/rust/range-patterns-dotdotdot, r=nick29581

This breaks code that looks like:

    match foo {
        1..3 => { ... }
    }

Instead, write:

    match foo {
        1...3 => { ... }
    }

Closes #17295.

r? @nick29581