Avoid dangling-pointer problem with partitionwise joins under GEQO.

build_child_join_sjinfo creates a derived SpecialJoinInfo in
the short-lived GEQO context, but afterwards the semi_rhs_exprs
from that may be used in a UniquePath for a child base relation.
This breaks the expectation that all base-relation-level structures
are in the planning-lifespan context, leading to use of a dangling
pointer with probable ensuing crash later on in create_unique_plan.
To fix, copy the expression trees when making a UniquePath.

Per bug #18360 from Alexander Lakhin.  This has been broken since
partitionwise joins were added, so back-patch to all supported
branches.

Discussion: https://postgr.es/m/18360-a23caf3157f34e62@postgresql.org

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