GRAM&ANN&TY: Support if let guard syntax

[mchernyavsky]

Closes #7794.

changelog: Support if let guard syntax

[Undin]

It would be great to support type inference and name resolution for items from if let guard.
For example, in the following example

#![feature(if_let_guard)]

enum Foo {
    Bar(i32),
    Baz
}

fn foo(f: Foo, y: i32) {
    match y {
        1 if let Foo::Bar(z) = f => { z + y }
        _ => 0
    };
}

the plugin knows neither the type nor location of z variable

Don't insist on implementing it in this PR. But probably it's worth converting the original issue into a meta issue for if let guard feature support and track progress there

[mchernyavsky]

@Undin fixed.

[Undin]

👍
bors r+

[bors]

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