The udl_{arg,formatter} structs are only defined when
FMT_USE_USER_DEFINED_LITERALS is set, so don't try to define things that
return that struct when it's not defined.

The intel compiler defines `__GNUC__`, but does not support the gcc `_Pragma`.  This PR filters out the intel compiler to avoid warnings about an unrecognized function.

 * do this  in _some_ tests to improve test coverage and catch possible problems due to that
 * fix invalid export of `static constexpr` constant
 * fix msvc warnings about unreachable code in high warning levels

UTF-8: https://datatracker.ietf.org/doc/html/rfc3629

Make just the necessary parts available for lookup from client context.