[OpenMP] Introduce the `ompx_dyn_cgroup_mem(<N>)` clause

Dynamic memory allows users to allocate fast shared memory when a kernel
is launched. We support a single size for all kernels via the
`LIBOMPTARGET_SHARED_MEMORY_SIZE` environment variable but now we can
control it per kernel invocation, hence allow computed values.

Note: Only the nextgen plugins will allocate memory based on the clause,
      the old plugins will silently miscompile.

Differential Revision: https://reviews.llvm.org/D141233