[flang][OpenMP] Consider previous DSA for static duration variables (#143601)
Symbols that have a pre-existing DSA set in the enclosing context should
not be made shared based on them being static duration variables.
Suggested-by: Leandro Lupori <leandro.lupori@linaro.org>
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Signed-off-by: Kajetan Puchalski <kajetan.puchalski@arm.com>