[msl-out] Handle struct padding in global variable initialiser

[hasali19]

WGSL:

struct Struct_1 {
    a: u32,
    b: vec3<u32>,
    c: i32,
}

var<private> x: Struct_1 = Struct_1(0u, vec3<u32>(0u, 0u, 0u), 0);

MSL:

// language: metal2.0
#include <metal_stdlib>
#include <simd/simd.h>

using metal::uint;

struct Struct_1_ {
    uint a;
    char _pad1[12];
    metal::packed_uint3 b;
    int c;
};
constant metal::uint3 const_type_1_ = {0u, 0u, 0u};
constant Struct_1_ const_Struct_1_ = {0u, const_type_1_, 0};

The initialiser for const_Struct_1_ ignores the padding bytes of Struct_1_.

The metal compiler complains with

naga.metal:14:43: error: cannot initialize an array element of type 'const constant char' with an lvalue of type 'const constant metal::uint3' (aka 'const constant uint3')
constant Struct_1_ const_Struct_1_ = {0u, const_type_1_, 0};
                                          ^~~~~~~~~~~~~

[teoxoy]

This actually looks like a duplicate of #870. Looks like the fix in #1215 didn't cover global constants.