[OPENMP50]Codegen for reduction clauses with 'task' modifier.

[OPENMP50]Codegen for reduction clauses with 'task' modifier.

Summary:
Added codegen for reduction clause with task modifier.
```
  #pragma omp ... reduction(task, +: a)
  {
  #pragma omp ... in_reduction(+: a)
  }
```
is translated into something like this:
```
  #pragma omp ... reduction(+:a)
  {
    struct red_input_t {
      void *reduce_shar;
      void *reduce_orig;
      size_t reduce_size;
      void *reduce_init;
      void *reduce_fini;
      void *reduce_comb;
      unsigned flags;
    } r_var;
    r_var.reduce_shar = &a;
    r_var.reduce_orig = &original a;
    r_var.reduce_size = sizeof(a);
    r_var.reduce_init = [](void* l,void*){return *(int*)l=0;};
    r_var.reduce_fini = nullptr;
    r_var.reduce_comb = [](void* l,void* r){return *(int*)l += *(int)r;};
    void *tg = __kmpc_taskred_modifier_init(<loc_addr>,<gtid>,
      <flag - 0 for parallel, 1 for worksharing>,
      <1 - number of reduction elements>,
      &r_var);
    {
    #pragma omp ... in_reduction(+: a) firstprivate(tg)
    ...
    }
    __kmpc_task_reduction_modifier_fini(<loc_addr>,<gtid>,
      <flag - 0 for parallel, 1 for worksharing>);
  }
```

Reviewers: jdoerfert

Subscribers: yaxunl, guansong, jfb, cfe-commits, caomhin

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79034