[OPENMP] Codegen for 'firstprivate' clause in 'task' directive.

[OPENMP] Codegen for 'firstprivate' clause in 'task' directive.

For tasks codegen for private/firstprivate variables are different rather than for other directives.

1. Build an internal structure of privates for each private variable:
struct .kmp_privates_t. {
  Ty1 var1;
  ...
  Tyn varn;
};
2. Add a new field to kmp_task_t type with list of privates.
struct kmp_task_t {
  void *              shareds;
  kmp_routine_entry_t routine;
  kmp_int32           part_id;
  kmp_routine_entry_t destructors;
  .kmp_privates_t.    privates;
};
3. Create a function with destructors calls for all privates after end of task region.
kmp_int32 .omp_task_destructor.(kmp_int32 gtid, kmp_task_t *tt) {
  ~Destructor(&tt->privates.var1);
  ...
  ~Destructor(&tt->privates.varn);
  return 0;
}
4. Perform initialization of all firstprivate fields (by simple copying for POD data, copy constructor calls for classes) + provide address of a destructor function after kmpc_omp_task_alloc() and before kmpc_omp_task() calls.
kmp_task_t *new_task = __kmpc_omp_task_alloc(ident_t *, kmp_int32 gtid, kmp_int32 flags, size_t sizeof_kmp_task_t, size_t sizeof_shareds, kmp_routine_entry_t *task_entry);

CopyConstructor(new_task->privates.var1, *new_task->shareds.var1_ref);
new_task->shareds.var1_ref = &new_task->privates.var1;
...
CopyConstructor(new_task->privates.varn, *new_task->shareds.varn_ref);
new_task->shareds.varn_ref = &new_task->privates.varn;

new_task->destructors = .omp_task_destructor.;
kmp_int32 __kmpc_omp_task(ident_t *, kmp_int32 gtid, kmp_task_t *new_task)
Differential Revision: http://reviews.llvm.org/D9370

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@236479 91177308-0d34-0410-b5e6-96231b3b80d8