[libclc] [NFC][libclc] Merge atomic extension built-ins with identical name into a single file (PR #134489)

[Wenju He via cfe-commits]

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@@ -6,6 +6,17 @@
 //
 //===----------------------------------------------------------------------===//
 
+// cl_khr_global_int32_base_atomics
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wenju-he wrote:

done, you're right that they should be all guarded since they are extensions. Thanks.

https://github.com/llvm/llvm-project/pull/134489