Offset/alignment for 0 bit types

[daurnimator]

e.g. If I have a packed struct with comptime parametized types, I don't want to have things not work when a field is passed as void/u0/struct{}. Instead, the offset should be right where it is (note that this differs from C where a 0-sized field aligns to the next byte boundary)

    const a = packed struct {
        foo: i32,
        bar: u0,
        baz: i32,
    };
    assert(@bitOffsetOf(a, "bar") == @bitOffsetOf(a, "baz"));

Another use case for this is helping to create structs with trailing data:

const mystruct = packed struct {
    foo: X,
    bar: Y,
    trailing_data:  @alignOf(Z) void,
}

const slice = try allocator.alignedAlloc(u8, @alignOf(mystruct), @sizeOf(mystruct) + extra_size);
const a = @ptrCast(*mystruct, slice.ptr);
const trailing_data = @ptrCast([*]Z, &a.trailing_data);

[SpexGuy]

With the decision in #6706, pointers to zero-sized values are now real pointers. This also means that offset and alignment will behave as described in this issue.

[andrewrk]

I believe this is fixed now and only task left is to ensure there is behavior test coverage.