Change global const handling to silently ignore redeclarations

test/mjsunit/regress/regress-1213575.js

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-// Make sure that a const definition always
-// conflicts with a defined setter. This avoid
-// trying to pass 'the hole' to the setter.
+// Make sure that a const definition does not try
+// to pass 'the hole' to a defined setter.
 
 this.__defineSetter__('x', function(value) { assertTrue(false); });
 
 var caught = false;
 try {
   eval('const x');
 } catch(e) {
   assertTrue(e instanceof TypeError);
   caught = true;
 }
-assertTrue(caught);
+assertFalse(caught);