[SYCL] Adding support for 'sycl_special_class' attribute.

clang/include/clang/Basic/Attr.td

@@ -1232,6 +1232,13 @@ def SYCLKernel : InheritableAttr {
   let Documentation = [SYCLKernelDocs];
 }
 
+def SYCLSpecialClass: InheritableAttr {
+  let Spellings = [Clang<"sycl_special_class">];
+  let Subjects = SubjectList<[CXXRecord]>;
+  let LangOpts = [SYCLIsDevice, SilentlyIgnoreSYCLIsHost];
+  let Documentation = [SYCLSpecialClassDocs];
+}
+
 // Marks functions which must not be vectorized via horizontal SIMT widening,
 // e.g. because the function is already vectorized. Used to mark SYCL
 // explicit SIMD kernels and functions.

clang/include/clang/Basic/AttrDocs.td

@@ -405,6 +405,26 @@ The SYCL kernel in the previous code sample meets these expectations.
   }];
 }
 
+def SYCLSpecialClassDocs : Documentation {
+  let Category = DocCatStmt;
+  let Content = [{
+The ``__attribute__((sycl_special_class))`` attribute is used in SYCL
+headers to indicate that a class or a struct needs additional handling when
+it is passed from host to device. Please note that this is an attribute that is
+used for internal implementation and not intended to be used by external users.
+It is used for ``accessor``, ``sampler`` , or ``stream`` classes.
+The types that own this attribute are excluded from device-copyable and other
+type-legalization steps.
+
+.. code-block:: c++
+  class __attribute__((sycl_special_class)) accessor {
+
+private:
+  void __init() {}
+};
+  }];
+}
+
 def SYCLSimdDocs : Documentation {
   let Category = DocCatFunction;
   let Content = [{

clang/lib/Sema/SemaDeclAttr.cpp

@@ -9782,6 +9782,9 @@ static void ProcessDeclAttribute(Sema &S, Scope *scope, Decl *D,
   case ParsedAttr::AT_SYCLSimd:
     handleSimpleAttribute<SYCLSimdAttr>(S, D, AL);
     break;
+  case ParsedAttr::AT_SYCLSpecialClass:
+    handleSimpleAttribute<SYCLSpecialClassAttr>(S, D, AL);
+    break;
   case ParsedAttr::AT_SYCLDevice:
     handleSYCLDeviceAttr(S, D, AL);
     break;