Fix invalid reads with strings

Fix invalid reads with strings

It may happen that a dynamically created compilation unit disappears before any
QV4::Strings it created. Those strings would still have a reference to the
QString data in the compilation unit. I don't see a choice other than making a
copy of the string data ;(. But this patch adds a flag that would allow for
avoiding it if we happen to know that the compilation unit data is static.

Change-Id: Ib35a4d2a566b301a25ffe56e392809e44e7b4ae8
Reviewed-by: Lars Knoll <lars.knoll@digia.com>