[Flang][MLIR][OpenMP] Temporarily re-add basic handling of uses in target...

[Flang][MLIR][OpenMP] Temporarily re-add basic handling of uses in target regions to avoid gfortran test-suite regressions

This was a regression introduced by myself in:

 https://github.com/llvm/llvm-project/commit/6a62707c048e16ce9bad37ed8e3520799139436b

where I too hastily removed the basic handling of implicit captures
we have currently. This will be superseded by all implicit captures
being added to target operations map_info entries in a soon landing
series of patches, however, that is currently not the case so we must
continue to do some basic handling of these captures for the time
being. This patch re-adds that behaviour to avoid regressions.

Unfortunately this means some test changes as well as
getUsedValuesDefinedAbove grabs constants used outside
of the target region which aren't handled particularly
well currently.