Fix Clang warning with __builtin_assume (#1788)

Building the project with `Clang` generates the following warning:
```
the argument to '__builtin_assume' has side effects that will be discarded [-Wassume]
```

Seems like Clang doesn't handle the hint well unless we pass a
const/pure attribute to the function call inside the assume. See:
llvm/llvm-project#55636 and
llvm/llvm-project#93077

With this change performance for the fn fused nabla examples is
significantly improved on clang-cuda and is now much faster than (our
old) nvcc, which doesn't have `assume` support.

include/gridtools/fn/unstructured.hpp

@@ -70,8 +70,9 @@ namespace gridtools::fn {
             int m_index;
         };
 
+        /// gnu::pure attribute is necessary to enable __builtin_assume() optimization with Clang
         template <class Tag, class Ptr, class Strides, class Domain>
-        GT_FUNCTION constexpr bool can_deref(iterator<Tag, Ptr, Strides, Domain> const &it) {
+        [[gnu::pure]] GT_FUNCTION constexpr bool can_deref(iterator<Tag, Ptr, Strides, Domain> const &it) {
             return it.m_index != -1;
         }