Adds support for printing the types char8_t, char16_t, and char32_t

This changes prints these types as Unicode code points. It is possible
that there is a better way of printing these types, but that change is
more complex, and the format in which Googletest prints these types is
subject to change if someone implements a better way of printing them.

This fixes the C++20 build, which removed support for printing these types.
https://en.cppreference.com/w/cpp/io/basic_ostream/operator_ltlt2

Fixes #2854

PiperOrigin-RevId: 314826912