Fix parsing of labeled loop followed by paren in stmt or match arm position

[dtolnay]

Previously, loop {} () was correctly parsed as:

Stmt::Expr(
    Expr::Loop {
        body: Block,
    },
    None,
),
Stmt::Expr(
    Expr::Tuple,
    None,
),

but 'a: loop {} () was parsed incorrectly:

Stmt::Expr(
    Expr::Call {
        func: Expr::Loop {
            label: Some(Label {
                name: Lifetime {
                    ident: "a",
                },
            }),
            body: Block,
        },
    },
    None,
),

They both need to be parsed as Expr::Loop followed by Expr::Tuple.