gv.c: For ‘Global symbol’ msg, don’t check utf8ness twice

We already take care of possible utf8ness in the format string.  There
is no need to turn on the flag afterwards.  This double utf8 check has
been present since ecad31f018.
Teach diag.t about Perl_mess

Every instance of Perl_mess(aTHX_ "..." ...) is used for an error
message.