fix issue 10720 - ICE with is(aaOfNonCopyableStruct.nonExistingField)

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.
Merge pull request #554 from 9rnsr/fix10720

issue 10720 - ICE with is(aaOfNonCopyableStruct.nonExistingField)
add explanatory comments
Merge pull request #555 from WalterBright/addSectionComments

add explanatory comments