issue 10720 - ICE with is(aaOfNonCopyableStruct.nonExistingField)

http://d.puremagic.com/issues/show_bug.cgi?id=10720

AA.dup should be disabled if Value type is not copyable.
The root cause was the use of Value.init. Built-in `init` property makes rvalue, therefore the 'static if' condition did not properly test 'copyable' trait of Value type.