[PATCH] [OPENMP] Codegen for 'private' clause in 'task' directive.

[Alexey Bataev]

Hi rjmccall, hfinkel,

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. Replace addresses of shared variables by the addresses of private ones in a structure of captured variables.
```
kmp_int32 .omp_task_entry.(kmp_int32 gtid, kmp_task_t *tt) {
  tt->shareds.var1_ref = &tt->privates.var1;
  ...
  tt->shareds.varn_ref = &tt->privates.varn;
  TaskFunction(gtid, tt->part_id, tt->shareds);
  return 0;
}
```
4. 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;
}

```
5. Perform default initialization of all private fields (no initialization for POD data, default 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);
DefaultConstructor(new_task->privates.var1);
...
DefaultConstructor(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)
```

http://reviews.llvm.org/D9322

Files:
  lib/CodeGen/CGOpenMPRuntime.cpp
  lib/CodeGen/CGOpenMPRuntime.h
  lib/CodeGen/CGStmtOpenMP.cpp
  test/OpenMP/task_private_codegen.cpp

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