[libclc] [libclc] Move mem_fence and barrier to clc library (PR #151446)

[Wenju He via cfe-commits]

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@@ -0,0 +1,37 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <clc/mem_fence/clc_mem_fence.h>
+
+void __clc_amdgcn_s_waitcnt(unsigned flags);
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wenju-he wrote:

> We should get rid of this; it can be properly implemented with `__builtin_amdgcn_fence`

I'll follow up

https://github.com/llvm/llvm-project/pull/151446