[MLIR][OpenMP] Support allocations of device shared memory

This patch updates the allocation of some reduction and private variables
within target regions to use device shared memory rather than private memory.
This is a prerequisite to produce working Generic kernels containing parallel
regions.

In particular, the following situations result in the usage of device shared
memory (only when compiling for the target device if they are placed inside of
a target region representing a Generic kernel):
- Reduction variables on `teams` constructs.
- Private variables on `teams` and `distribute` constructs that are reduced or
used inside of a `parallel` region.

Currently, there is no support for delayed privatization on `teams` constructs,
so private variables on these constructs won't currently be affected. When
support is added, if it uses the existing `allocatePrivateVars` and
`cleanupPrivateVars` functions, usage of device shared memory will be
introduced automatically.

Author: skatrak