[SYCL] Implement local_accessor begin/end (#6692)

SYCL2020 introduces various accessor begin/end member functions that
allow user to iterate through the underlying buffer.

E2E tests: intel/llvm-test-suite#1225

Author: KornevNikita

sycl/include/sycl/accessor.hpp

@@ -2529,6 +2529,12 @@ class __SYCL_SPECIAL_CLASS __SYCL_TYPE(local_accessor) local_accessor
   // Use base classes constructors
   using local_acc::local_acc;
 
+  using value_type = DataT;
+  using iterator = value_type *;
+  using const_iterator = const value_type *;
+  using reverse_iterator = std::reverse_iterator<iterator>;
+  using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+
 #ifdef __SYCL_DEVICE_ONLY__
 
   // __init needs to be defined within the class not through inheritance.
@@ -2550,6 +2556,24 @@ class __SYCL_SPECIAL_CLASS __SYCL_TYPE(local_accessor) local_accessor
 #endif
 
 public:
+  iterator begin() const noexcept {
+    return &this->operator[](id<Dimensions>());
+  }
+  iterator end() const noexcept { return begin() + this->size(); }
+
+  const_iterator cbegin() const noexcept { return const_iterator(begin()); }
+  const_iterator cend() const noexcept { return const_iterator(end()); }
+
+  reverse_iterator rbegin() const noexcept { return reverse_iterator(end()); }
+  reverse_iterator rend() const noexcept { return reverse_iterator(begin()); }
+
+  const_reverse_iterator crbegin() const noexcept {
+    return const_reverse_iterator(end());
+  }
+  const_reverse_iterator crend() const noexcept {
+    return const_reverse_iterator(begin());
+  }
+
   template <typename Property> bool has_property() const noexcept {
 #ifndef __SYCL_DEVICE_ONLY__
     return this->getPropList().template has_property<Property>();