Add Perl_ck_warner(), which combines Perl_ckwarn() and Perl_warner().

Add Perl_ck_warner(), which combines Perl_ckwarn() and Perl_warner().

Replace ckWARN{,2,3,4}() && Perl_warner() with it, which trades reduced code
size (about 0.2%), for 1 more function call if warnings are not enabled.
However, if we're now in the L1 or L2 cache when we weren't previously, that's
still going to be a speed win.