Lowering of unions with nested structs sometimes incorrect with capabilities

[arichardson]

Structures like the following:

union {
    struct {
        int a;
        __intcap b;
    } c;
    struct {
        __intcap d;
        int e;
    } f;
};

are lowered to IR the same way as one of the struct members, i.e. { i32, ptr addrspace(200) } even though capabilities could exist at offset zero and 8/16.

Unlikely to result in tag-stripping in practice since those unions will be passed indirectly, but could be a problem once 2*CLEN structures are passed in registers.

See #723

[jrtc27]

Well, SROA would likely have fun with breaking them