[OPENMP50]Initial codegen for 'affinity' clauses.

Summary:
Added initial codegen for 'affinity' clauses on task directives.
Emits next code:
```
kmp_task_affinity_info_t affs[<num_elems>];

void *td = __kmpc_task_alloc(..);

affs[<i>].base = &data_i;
affs[<i>].size = sizeof(data_i);
__kmpc_omp_reg_task_with_affinity(&loc, <gtid>, td, <num_elems>, affs);
```

The result returned by the call of `__kmpc_omp_reg_task_with_affinity`
function is ignored currently sincethe  runtime currently ignores args
and returns 0 uncoditionally.

Reviewers: jdoerfert

Subscribers: yaxunl, guansong, sstefan1, llvm-commits, cfe-commits, caomhin

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D80240