[SYCL] Allow pre-declared C++ classes to be used without being defined

Signed-off-by: Vladimir Lazarev <vladimir.lazarev@intel.com>

clang/lib/Sema/SemaSYCL.cpp

@@ -250,6 +250,12 @@ class MarkDeviceFunction : public RecursiveASTVisitor<MarkDeviceFunction> {
       Ty = QualType{Ty->getPointeeOrArrayElementType(), 0};
 
     if (const auto *CRD = Ty->getAsCXXRecordDecl()) {
+      // FIXME: this seems like a temporary fix for SYCL programs
+      // that pre-declare, use, but not define OclCXX classes,
+      // which are later translated into SPIRV types.
+      if(!CRD->hasDefinition())
+        return true;
+
       if (CRD->isPolymorphic()) {
         SemaRef.Diag(CRD->getLocation(), diag::err_sycl_virtual_types);
         SemaRef.Diag(Loc.getBegin(), diag::note_sycl_used_here);