Avoid u8"" literals in tests, their type changes in C++20

Just specify the encoded bytes instead.
Additionally delete insertion operator of raw_ostream for char8_t as it
doesn't work as users might expect (Numbers and pointers are added to
the stream instead of UTF-8 characters). Added a comment and instructions
on how to use UTF-8 strings with raw_ostream.

Differential Revision: https://reviews.llvm.org/D141392