Support interpolated $:path followed by macro call or struct literal

[dtolnay]

Example of an input that previously behaved incorrectly:

let nested = Group::new(Delimiter::None, quote!(a::b || true));
let tokens = quote!(if #nested && false {});
syn::parse2::<syn::Expr>(tokens).unwrap()

Before: this has the wrong precedence. a::b || (true && false)

Expr::If {
    cond: Expr::Binary {
        left: Expr::Path {
            path: Path {
                segments: [
                    PathSegment {
                        ident: "a",
                    },
                    PathSegment {
                        ident: "b",
                    },
                ],
            },
        },
        op: BinOp::Or,
        right: Expr::Binary {
            left: Expr::Lit {
                lit: Lit::Bool {
                    value: true,
                },
            },
            op: BinOp::And,
            right: Expr::Lit {
                lit: Lit::Bool {
                    value: false,
                },
            },
        },
    },
    then_branch: Block {
        stmts: [],
    },
}

After: correct precedence. (a::b || true) && false

Expr::If {
    cond: Expr::Binary {
        left: Expr::Group {
            expr: Expr::Binary {
                left: Expr::Path {
                    path: Path {
                        segments: [
                            PathSegment {
                                ident: "a",
                            },
                            PathSegment {
                                ident: "b",
                            },
                        ],
                    },
                },
                op: BinOp::Or,
                right: Expr::Lit {
                    lit: Lit::Bool {
                        value: true,
                    },
                },
            },
        },
        op: BinOp::And,
        right: Expr::Lit {
            lit: Lit::Bool {
                value: false,
            },
        },
    },
    then_branch: Block {
        stmts: [],
    },
}