Print std::u8string, std::u16string, and std::u32string as string literals

Previously, these types were printed as "{ U+123, U+456, U+789 }". However,
printed output in that form is difficult to compare against any literals that
might be defined in code. Instead, just treat these types like std::string
and std::wstring, escaping non-ASCII characters with a hexadecimal escape
sequence.

The tests have also been updated to cover the new functionality: as a bonus,
the tests now also pass with the MSVC toolchain.

Internally, the code has been reorganized to primarily operate in terms of
char32_t, under the assumption that char32_t will always be at least as big
as wchar_t. While that assumption is currently true, perhaps it won't be in
the future...

PiperOrigin-RevId: 364033132