[OPENMP] Prepare codegen for privates in tasks for non-capturing of privates in CapturedStmt.

Reworked codegen for privates in tasks:

call @kmpc_omp_task_alloc();
...
call @kmpc_omp_task(task_proxy);

void map_privates(.privates_rec. *privs, type1 ** priv1_ref, ..., typen **privn_ref) {
  *priv1_ref = &privs->private1;
  ...
  *privn_ref = &privs->privaten;
  ret void
}

i32 task_entry(i32 ThreadId, i32 PartId, void* privs, void (void*, ...) map_privates, shareds* captures) {
  type1 **priv1;
  ...
  typen **privn;
  call map_privates(privs, priv1, ..., privn);
  <Task body with priv1, .., privn instead of the captured variables>.
  ret i32
}

i32 task_proxy(i32 ThreadId, kmp_task_t_with_privates *tt) {
  call task_entry(ThreadId, tt->task_data.PartId, &tt->privates, map_privates, tt->task_data.shareds);
}

llvm-svn: 238010