Allow COW PVMGs to be tied

Allow COW PVMGs to be tied

This logic in sv_magic is wrong:

    if (SvREADONLY(sv)) {
	if (
	    /* its okay to attach magic to shared strings; the subsequent
	     * upgrade to PVMG will unshare the string */
	    !(SvFAKE(sv) && SvTYPE(sv) < SVt_PVMG)

	    && IN_PERL_RUNTIME
	    && !PERL_MAGIC_TYPE_READONLY_ACCEPTABLE(how)
	   )
	{
	    Perl_croak_no_modify(aTHX);
	}
    }

There is nothing wrong with attaching magic to a shared string that
will stay shared.  Also, shared strings are not always < SVt_PVMG.
 Sometimes a PVMG or PVLV can end up with a shared string.  In those
cases, the logic above treats them as read-only, which they ain’t.

The easiest example is a downgraded typeglob:

$x = *foo;        # now a PVGV
undef $x ;        # downgraded to PVMG
$x = __PACKAGE__; # now a shared string (COW)
tie $x, "main";   # bang! $x is considered read-only
sub main::TIESCALAR{bless[]}