Try: Mark some parts of task.h as constexpr for clang 21.1

Author: WardBrian

lib/tbb_2020.3/STAN_CHANGES

@@ -13,4 +13,5 @@ This file documents changes done for the stan-math project
   - build/windows.gcc.inc
     - L84 Wrapped the use of `-flifetime-dse` flag in conditional on non-WINARM64
     - L101 Wrapped the use of `-msse` in conditional on non-WINARM64
-    
+
+- Add `constexpr` to lines 589-593 of `task.h` to fix a build failure in Clang 21.1

lib/tbb_2020.3/include/tbb/task.h

@@ -586,10 +586,10 @@ class task_group_context : internal::no_copy {
     friend class task;
     friend class internal::allocate_root_with_context_proxy;
 
-    static const kind_type binding_required = bound;
-    static const kind_type binding_completed = kind_type(bound+1);
-    static const kind_type detached = kind_type(binding_completed+1);
-    static const kind_type dying = kind_type(detached+1);
+    static constexpr kind_type binding_required = bound;
+    static constexpr kind_type binding_completed = kind_type(bound+1);
+    static constexpr kind_type detached = kind_type(binding_completed+1);
+    static constexpr kind_type dying = kind_type(detached+1);
 
     //! Propagates any state change detected to *this, and as an optimisation possibly also upward along the heritage line.
     template <typename T>