implement field alignment syntax for unions

[andrewrk]

Thanks to #3114 we now have syntax for specifying alignment on struct and union fields. Once #3115 is merged, the syntax for struct field alignment will work (#1512). However there is more work to do to implement support for this syntax for unions, and this issue is to track the implementation. Here's a test case:

const std = @import("std");
const assert = std.debug.assert;

const Number = union {
    int: u8 align(64),
    float: f64,
};

test "union field alignment syntax" {
    var num =  Number {.float  = 12.34};
    assert(num.float == 12.34);
    num = Number {.int = 100};
    assert(num.int == 100);
    assert(@typeOf(&num.int) == *align(64) u8);
    assert(@ptrToInt(&num.int) % 64 == 0);
}

[hnakamur]

I found another reproducer for this.
It is a union typedef in C like below:

typedef union
{
    uint64_t f1;
    uint8_t f2[0];
} my_union;

Please refer to https://github.com/hnakamur/zig-c-union-align-compile-error-repro for the full reproducer.