Wrong alignment for struct fields on x86_64/amd64

[x539]

When using align in structs, the .offsetof-property of fields is correct but real alignment is wrong:

$ ldc2 --version
LDC - the LLVM D compiler (0.12.0):
based on DMD v2.063.2 and LLVM 3.3
Default target: x86_64-redhat-linux-gnu
Host CPU: corei7

import std.stdio;

struct buggy {
        align(1):
        uint a;
        ulong b;
}

void main() {
        union uni {
                ubyte[buggy.sizeof] raw;
                buggy struc;
        };

        uni data;

        // This is OK.
        static assert(buggy.b.offsetof == uint.sizeof);

        data.struc.b=0x0807060504030201;

        writeln(buggy.b.offsetof);

        // Print the real data layout, should be 4 zeros followed by 1,2..8
        writeln(data.raw);

        // This fails due to bad alignment of b as you can see above
        assert(cast(ubyte)(data.struc.b) == data.raw[buggy.b.offsetof]);

}

[dnadlinger]

Confirmed on Git master. The issue is that we fail to mark the struct as packed on the LLVM IR level. As a workaround, you can use align(1) struct buggy { … } for the time being.

[dnadlinger]

Fixed by #741.