Allow to distinguish wide strings from regular strings.

[emilio]

#define FOO L"bar"

Produces the same output as:

#define FOO "bar"

While they're different types. This causes bindgen to not generate the right thing for the former.

[paxbun]

I have a similar issue due to this. If you think it's okay, I want to make a PR that adds enum variants Char8, Char16, Char32, and WChar, which correspond to the u8, u, U, and L prefixes.