[clang] [OpenCL] Add missing OpenCL 3.0 features to OpenCLExtensions.def; revert header-only macros (PR #168016)

[Sven van Haastregt via cfe-commits]

================
@@ -234,6 +234,11 @@ C23 Feature Support
 
 Non-comprehensive list of changes in this release
 -------------------------------------------------
+- Removed OpenCL header-only feature macros (previously unconditionally enabled
+  on SPIR-V and only selectively disabled via -D__undef_<feature>). All OpenCL
----------------
svenvh wrote:

```suggestion
  on SPIR-V and only selectively disabled via ``-D__undef_<feature>``). All OpenCL
```

https://github.com/llvm/llvm-project/pull/168016