Ensure cl.xml passes validation

[alycm]

Ensure cl.xml passes validation, and add validation to CI to avoid regressions.

Some issues were fixed by changing cl.xml, some by changing registry.rnc. I tested that XML changes didn't break anything by ensuring the generated spec doesn't change, and that the re-generated extension headers didn't change.

I tested the CI change by leaving a validation in the XML at first and ensuring that the build failed.

Fixes #830

[alycm]

I flip-flopped on whether this should be a schema change or an XML change. I landed on schema change because the single use of this pattern does seem valid.

It is for clEnqueueMigrateMemINTEL within cl_intel_unified_shared_memory which requires="CL_VERSION_1_2". In other cases it's the entire extension that requires an OpenCL version, but this instance is specified at a finer granularity.

[bashbaug]

Yeah, I remember discussing various options for clEnqueueMigrateMemINTEL and settling on this as the best option, though I can't find the discussion in an issue so it may have been in a PR comment. I agree this could be a useful pattern going forward for other extensions, as well.

[SunSerega]

Can't see a discussion on this specifically, but here you say you settled on this to get things un-stuck.
And the requires="CL_VERSION_1_2" didn't exist on clEnqueueMigrateMemINTEL before #290 - checked with git blame.