[OpenCL] Restore ATOMIC_VAR_INIT

We accidentally lost the ATOMIC_VAR_INIT and ATOMIC_FLAG_INIT macros
in r363794.

Also put the `memory_order` typedef back inside a `>= CL2.0` guard.

llvm-svn: 364174
diff --git a/clang/lib/Headers/opencl-c-base.h b/clang/lib/Headers/opencl-c-base.h
index 19aaa90..a82954d 100644
--- a/clang/lib/Headers/opencl-c-base.h
+++ b/clang/lib/Headers/opencl-c-base.h
@@ -297,8 +297,11 @@
  * image memory.
  */
 #define CLK_IMAGE_MEM_FENCE  0x04
-#endif //__OPENCL_C_VERSION__ >= CL_VERSION_2_0
 
+#ifndef ATOMIC_VAR_INIT
+#define ATOMIC_VAR_INIT(x) (x)
+#endif //ATOMIC_VAR_INIT
+#define ATOMIC_FLAG_INIT 0
 
 // enum values aligned with what clang uses in EmitAtomicExpr()
 typedef enum memory_order
@@ -310,6 +313,8 @@
   memory_order_seq_cst = __ATOMIC_SEQ_CST
 } memory_order;
 
+#endif //__OPENCL_C_VERSION__ >= CL_VERSION_2_0
+
 // OpenCL v1.1 s6.11.3, v1.2 s6.12.14, v2.0 s6.13.14 - Image Read and Write Functions
 
 // These values need to match the runtime equivalent