[OPENMP50]Codegen for uses_allocators clause.

Summary:
Predefined allocators should not be mapped at all (they are just enumeric
constants). FOr user-defined allocators need to map the traits only as
firstprivates, the allocator itself is private.
At the beginning of the target region the user-defined allocatores must
be created and then destroyed at the end of the target region:
```
omp_allocator_handle_t my_allocator = __kmpc_init_allocator(<gtid>,
/*default memhandle*/ 0, <number_of_traits>, &<traits>);
...
call void @__kmpc_destroy_allocator(<gtid>, my_allocator);
```

Reviewers: jdoerfert, aaron.ballman

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

Tags: #clang

Differential Revision: https://reviews.llvm.org/D79257