stage2 llvm: fix handling of pointer fields in packed structs

[Vexu]

This makes Aro pass all tests with #11798 worked around and #11867 applied.

[andrewrk]

I think we can merge this since it is an improvement over status quo, which produces an invalid LLVM module, although I would like to improve the test case to actually check that the code is correctly compiled rather than only checking that it does not crash the compiler.

Note however that we do have some more work to do with pointers to packed structs:

const Symbol = struct {
    index: u16,
};

const Relocation = packed struct {
    symbol: *Symbol,
    a: u32,
};

var x: Relocation = .{
    .symbol = &s,
    .a = 1234,
};

var s: Symbol = undefined;

test "global packed struct with pointer field" {
    _ = x;
}

Invalid bitcast
i64 bitcast (%test3.Symbol* @test3.s to i64)

thread 81531 panic: LLVM module verification failed

In some cases it is impossible to lower code like this because there is no such relocation supported by the object file. However, I think we can allow it when it does work, as long as we can detect when it won't work and emit a compile error.