[libclc] [libclc] Add OpenCL atomic_*_explicit builtins (PR #168318)

[Wenju He via cfe-commits]

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@@ -28,7 +28,7 @@ function(compile_to_bc)
   get_filename_component( FILE_EXT ${ARG_INPUT} EXT )
   if( NOT ${FILE_EXT} STREQUAL ".ll" )
     # Pass '-c' when not running the preprocessor
-    set( PP_OPTS -c )
+    set( PP_OPTS -c -include opencl-c-base.h )
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wenju-he wrote:

> Maybe should be using -finclude-default-header or -fdeclare-opencl-builtin instead?
> 
> I'm assuming this picks up the clang builtin copy, not the duplicated name one in libclc?

Done, changed to `-Xclang -fdeclare-opencl-builtins -Xclang -finclude-default-header` in https://github.com/llvm/llvm-project/pull/168318/commits/58ddca51a771a93306742ae4cd03be1a006d27a1

The flag will pull in builtin declarations from clang, so the OpenCL builtin declarations in libclc are not needed anymore. I'll remove them in a separate PR.

https://github.com/llvm/llvm-project/pull/168318