Stop ck_rvconst from treating GV constants as strings

Stop ck_rvconst from treating GV constants as strings

sub foo { 42 }
use constant bar => *foo;
BEGIN { undef *foo }
warn &{+bar};
warn bar->();

Obviously the last two lines should print the same thing, because they
both call the value of the ‘bar’ constant as a suroutine.

But op.c:ck_rvconst messes up the ‘bar->()’ at compile time, treating
the bar glob (a copy of the original *foo glob, and not the *foo glob
itself, which has since been undefined) as a string and using it to
look up a glob.

ck_rvconst should not do anything if the constant’s value is a glob.