Provide symbolic name for extensions

[kpet]

Vulkan provides preprocessor macros (e.g. VK_KHR_SURFACE_EXTENSION_NAME defined as "VK_KHR_surface"). This helps with avoiding typos in extension names that can lead to silly bugs. We should do the same. These definitions could be generated automatically from the XML.

[bashbaug]

Note, we currently don't include extensions with no API changes in cl_ext.h. For example, cl_khr_3d_image_writes isn't in cl_ext.h. If we enable symbolic names for extensions, would we want to include all extension in cl_ext.h, even if it's only the symbolic name for the extension?

I think we would. I think this also means that we would want to include all extensions in the XML file also, if we're using it to generate cl_ext.h.