[OpenMP] Add an option to limit shared memory usage in OpenMPOpt

[OpenMP] Add an option to limit shared memory usage in OpenMPOpt

One of the optimizations performed in OpenMPOpt pushes globalized
variables to static shared memory. This is preferable to keeping the
runtime call in all cases, however if too many variables are pushed to
hared memory the kernel will crash. Since this is an optimization and
not something the user specified explicitly, there should be an option
to limit this optimization in those cases. This path introduces the
`-openmp-opt-shared-limit=` option to limit the amount of bytes that
will be placed in shared memory from HeapToShared.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D120079