Make overloaded constants always read-only

They were not read-only if the constant handler returned a shared
hash key scalar.

This was brought up in bug #109744.

lib/overload.t

@@ -1301,7 +1301,6 @@ foreach my $op (qw(<=> == != < <= > >=)) {
 	'constant overloading makes read-only constants';
     BEGIN { overload::constant integer => sub { __PACKAGE__ }; }
     eval { ${\5} = 'whatever' };
-    local $::TODO = ' ';
     like $@, qr/^Modification of a read-only value attempted at /,
 	'... even with shared hash key scalars';
 }

op.c

@@ -10562,7 +10562,7 @@ Perl_ck_svconst(pTHX_ OP *o)
 {
     PERL_ARGS_ASSERT_CK_SVCONST;
     PERL_UNUSED_CONTEXT;
-    if (!SvIsCOW(cSVOPo->op_sv)) SvREADONLY_on(cSVOPo->op_sv);
+    SvREADONLY_on(cSVOPo->op_sv);
     return o;
 }