[clang] [Clang][OpenCL] Promote a few extensions to OpenCL 3.1 core (PR #204330)

[Wenju He via cfe-commits]

wenju-he wrote:

> I don't think we should do this on a per-function level. Instead, we should mark the extensions accordingly in `clang/include/clang/Basic/OpenCLExtensions.def` I think.

done, thanks. Current implementation of macros in OpenCLExtensions.def seems to be making features unconditionally available if pragma is not required. But this is a separate issue, will look closely later.

https://github.com/llvm/llvm-project/pull/204330