ty: Use underlying layout of typedefs if available.

[emilio]

This fixes struct layout issues with over-aligned typedefs, which rust can't represent, see #3449 for context and some other discussion.

This is kinda ugly tho, but as far as I can tell there's no good way of getting the right ABI and struct layout at the same time...

[emilio]

cc @ojeda

[ojeda]

A few more cases for your tests/future considerations:

struct Inner { long long a, b; };
typedef struct Inner AlignedInner __attribute__((aligned(16)));

struct Outer {
    long long before;
    AlignedInner inner[1];
};

(and the same with --opaque-type AlignedInner).

struct Inner { long long a, b; };
typedef struct Inner AlignedInner __attribute__((aligned(16)));
struct Outer {
    long long before;
    AlignedInner inner[0];
    char tail;
};

enum __attribute__((aligned(16))) AlignedEnum { Value = 1 };
struct Outer {
    long long before;
    enum AlignedEnum inner;
};

typedef unsigned int U __attribute__((aligned(2)));
struct Outer {
    char before;
    U bits : 31;
    char mid;
    U inner;
    char tail;
};

All these changed w.r.t. 0.72.1.

I hope that helps & thanks!