Generate OpenCL C feature dictionary

The macros for OpenCL C features are currently defined manually in `c/feature-dictionary.asciidoc`. Here is an example:

```
// opencl_c_kernel_clock_scope_sub_group
ifdef::backend-html5[]
:opencl_c_kernel_clock_scope_sub_group: pass:q[`\__opencl_c_kernel_clock_scope_sub_group`]
endif::[]
ifndef::backend-html5[]
:opencl_c_kernel_clock_scope_sub_group: pass:q[`\__opencl_c_​kernel_​clock_​scope_​sub_​group`]
endif::[]
```

Authoring these manually is tedious and error prone. They should be generated from the XML.

Start by inspecting c/feature-dictionary.asciidoc and the XML definitions for OpenCL C features, then trace the documentation-generation workflow that consumes them. The work is done when the feature macros are generated from the XML rather than maintained manually and the resulting dictionary preserves the documented HTML and non-HTML forms.

Written by the indexing model from the issue text.