Adds support for wide, UTF-16, and UTF-32 string literals. If used to initialize
an incomplete array, the same logic as narrow strings is used. Otherwise they
are translated as global "anonymous" arrays of the relevant underlying char type.
A dot is used in the name to ensure the generated names do not conflict with any
other names in the translated program.

For example:

```c
void my_fn() {
    const uint32_t *foo = U"foo";
}
```

becomes:
```zig
const @"zig.UTF32_string_2" = [4]c_uint{
    '\u{66}',
    '\u{6f}',
    '\u{6f}',
    0,
};
pub export fn my_fn() void {
    var foo: [*c]const u32 = &@"zig.UTF32_string_2";
}
```