[clang] `-Wcharacter-conversion` should warn on `wchar_t` conversions

https://compiler-explorer.com/z/6zb99Y8Yj

void f(char8_t c8, char16_t c16, wchar_t cw) {
    bool a = c8 == c16;
    bool b = c8 == cw;
}

Currently, only the first line raises -Wcharacter-conversion; the second line doesn't.

However, a comparison between char8_t and wchar_t is just as nonsensical as a comparison between char8_t and char16_t, so this seems like a false negative. That is especially true when considering that wchar_t is often UTF-16 or UTF-32, depending on the platform.

Also note that when seeing the the corresponding paper P3695R2, SG16 Unicode unanimously voted to deprecate conversions between char8_t and wchar_t. We have since flipped on that decision because it is hypothetically possible to have a UTF-8 wchar_t in C2y, but the compiler doesn't really need to care about such far-fetched hypotheticals; it has platform knowledge.

CC @cor3ntin

Reproduce the Compiler Explorer example with Clang and compare the diagnostics for char8_t/char16_t and char8_t/wchar_t under -Wcharacter-conversion. Trace the Clang diagnostic path for the existing character-conversion warning, then add coverage for the wchar_t comparison. Done means the second comparison warns consistently with the first across relevant platforms.