[OPENMP50]Initial support for detach clause in task directive.

Added parsing/sema/serialization support for detach clause.
diff --git a/clang/lib/CodeGen/CGStmtOpenMP.cpp b/clang/lib/CodeGen/CGStmtOpenMP.cpp
index e3dd490..74fabb1 100644
--- a/clang/lib/CodeGen/CGStmtOpenMP.cpp
+++ b/clang/lib/CodeGen/CGStmtOpenMP.cpp
@@ -4615,6 +4615,7 @@
   case OMPC_nontemporal:
   case OMPC_order:
   case OMPC_destroy:
+  case OMPC_detach:
     llvm_unreachable("Clause is not allowed in 'omp atomic'.");
   }
 }