Fix T84745: more build errors with TBB 2021

* tbb::blocked_range moved to a different namespace and since the fix is
  non-trivial, remove some unused code that used this.
* Task group priorities are no longer supported. It's unclear if they are
  useful at all right now and even set correctly, for now all tasks are equal
  priority with TBB 2021.

intern/cycles/util/util_tbb.h

@@ -36,7 +36,14 @@ using tbb::parallel_for;
 
 static inline void parallel_for_cancel()
 {
+#if TBB_INTERFACE_VERSION_MAJOR >= 12
+  tbb::task_group_context *ctx = tbb::task::current_context();
+  if (ctx) {
+    ctx->cancel_group_execution();
+  }
+#else
   tbb::task::self().cancel_group_execution();
+#endif
 }
 
 CCL_NAMESPACE_END

source/blender/blenlib/BLI_index_range.hh

@@ -58,11 +58,6 @@
 
 #include "BLI_utildefines.h"
 
-/* Forward declare tbb::blocked_range for conversion operations. */
-namespace tbb {
-template<typename Value> class blocked_range;
-}
-
 namespace blender {
 
 template<typename T> class Span;
@@ -86,12 +81,6 @@ class IndexRange {
     BLI_assert(size >= 0);
   }
 
-  template<typename T>
-  constexpr IndexRange(const tbb::blocked_range<T> &range)
-      : start_(range.begin()), size_(range.size())
-  {
-  }
-
   class Iterator {
    private:
     int64_t current_;

source/blender/blenlib/intern/task_pool.cc

@@ -131,6 +131,12 @@ class TBBTaskGroup : public tbb::task_group {
  public:
   TBBTaskGroup(TaskPriority priority)
   {
+#  if TBB_INTERFACE_VERSION_MAJOR >= 12
+    /* TODO: support priorities in TBB 2021, where they are only available as
+     * part of task arenas, no longer for task groups. Or remove support for
+     * task priorities if they are no longer useful. */
+    UNUSED_VARS(priority);
+#  else
     switch (priority) {
       case TASK_PRIORITY_LOW:
         my_context.set_priority(tbb::priority_low);
@@ -139,6 +145,7 @@ class TBBTaskGroup : public tbb::task_group {
         my_context.set_priority(tbb::priority_normal);
         break;
     }
+#  endif
   }
 
   ~TBBTaskGroup()