Allow enums with C compatible tag types in extern structs/functions

[Hejsil]

As seen in #977, the tag types of enums in the C spec is unreliable. @isaachier even points out that many C projects avoid the C enum so they can ensure that their lib is "ABI-able".

But Zig has explicit tag types for enums, so when fixing up code translated with translate-c, we should be able to replace code such as this:

typedef E uint32_t;
enum { A, B, C };

struct S {
    E e;
};

With:

pub const E = enum(u32) { A, B, C };
pub const S =  extern struct {
    e: E,
};

Sadly, this is not allowed:

test.zig:3:5: error: extern structs cannot contain fields of type 'E'
    e: E,