compiler crash when accessing definition in extern union with 0 size

[tgschultz]

const std = @import("std");

test "0-sized extern union definition" {
    const U = extern union {
        a: void,
        const f = 1;
    };
    
    std.testing.expect(U.f == 1);
}

further testing reveals that the definition must be const to trigger the crash.