[SYCL] Add support for 'sycl_special_class' attribute. (#3892)

Special classes such as accessor, sampler, and stream need additional implementation when they are passed from host to device. Currently the compiler recognizes these classes by their name. 
This patch is adding a new attribute “sycl_special_class”  used to mark SYCL classes that need the additional compiler handling.
Usage:
```c++
class __attribute__((sycl_special_class)) class  special_class {
...
};
```

Signed-off-by: Zahira Ammarguellat <zahira.ammarguellat@intel.com>

Author: zahiraam

clang/include/clang/Basic/Attr.td

@@ -1233,6 +1233,13 @@ 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

@@ -9796,6 +9796,9 @@ 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;