segfault when passing union enum with u2 field to const slice parameter

[andrewrk]

reported by @thejoshwolfe in the segfault branch which I am now deleting, since I created a minimal reproducer:

pub const Id = union(enum) {
    Invalid: bool,
    StringLiteral: u2,
};

export fn entry() {
    testTokenize([]Id {
        Id { .StringLiteral = 1 },
    });
}

fn testTokenize(expected_tokens: []const Id) { }

pub fn panic(msg: []const u8) -> noreturn { while (true) {} }