[OPENMP50]Add parsing/sema analysis for nontemporal clause.

Add basic support for parsing/sema analysis of the nontemporal clause in
simd-based directives.
diff --git a/clang/lib/CodeGen/CGStmtOpenMP.cpp b/clang/lib/CodeGen/CGStmtOpenMP.cpp
index 3c266a5..e8a3790 100644
--- a/clang/lib/CodeGen/CGStmtOpenMP.cpp
+++ b/clang/lib/CodeGen/CGStmtOpenMP.cpp
@@ -4206,6 +4206,7 @@
   case OMPC_atomic_default_mem_order:
   case OMPC_device_type:
   case OMPC_match:
+  case OMPC_nontemporal:
     llvm_unreachable("Clause is not allowed in 'omp atomic'.");
   }
 }