[OPENMP 4.5] Fixed codegen for 'priority' and destructors in task-based

[OPENMP 4.5] Fixed codegen for 'priority' and destructors in task-based
directives.

'kmp_task_t' record type added a new field for 'priority' clause and
changed the representation of pointer to destructors for privates used
within loop-based directives.
Old representation:

typedef struct kmp_task {                   /* GEH: Shouldn't this be
aligned somehow? */
  void *shareds;                            /**< pointer to block of
    pointers to shared vars   */
  kmp_routine_entry_t routine;              /**< pointer to routine
    to call for executing task */
  kmp_int32 part_id;                        /**< part id for the
    task                          */
  kmp_routine_entry_t destructors;        /* pointer to function to
  invoke deconstructors of firstprivate C++ objects */
  /*  private vars  */
} kmp_task_t;

New representation:

typedef struct kmp_task {                   /* GEH: Shouldn't this be
aligned somehow? */
  void *shareds;                            /**< pointer to block of
    pointers to shared vars   */
  kmp_routine_entry_t routine;              /**< pointer to routine
    to call for executing task */
  kmp_int32 part_id;                        /**< part id for the
    task                          */
  kmp_cmplrdata_t data1; /* Two known
optional additions: destructors and priority */
  kmp_cmplrdata_t data2; /* Process
destructors first, priority second */
/* future data */
  /*  private vars  */
} kmp_task_t;

Also excessive initialization of 'destructors' fields to 'null' was
removed from codegen if it is known that no destructors shal be used.
Currently a special bit is used in 'kmp_tasking_flags_t' bitfields
('destructors_thunk' bitfield).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@271201 91177308-0d34-0410-b5e6-96231b3b80d8