fix: decoding struct and union fields with ambiguous types

[t0rr3sp3dr0]

The @" sequence is ambiguous when used inside structs and unions. This patch makes go-macho correctly decode these types based on the behaviour of clang.

Encoded Type	Decoded Type
{S="Id"@"NSString"^{S}}	struct S { id Id; struct S *NSString; }
{S="Id"@"NSString""NSString"^{S}}	struct S { NSString *Id; struct S *NSString; }
{?=@"NSString"^{S}}	struct { NSString *; struct S *; }

Before:

@class next;

@interface __CFPrefsWeakObservers : NSObject <NSMutableCopying> {
    /* instance variables */
    struct __cfobservers_t { next *slot; struct __cfobservers_t *x1; } values;

After:

@interface __CFPrefsWeakObservers : NSObject <NSMutableCopying> {
    /* instance variables */
    struct __cfobservers_t { id slot; struct __cfobservers_t *next; } values;