[mlir][spirv] Use separate attribute for (version, capabilities, extensions)

We also need the (version, capabilities, extensions) triple on the
spv.module op. Thus far we have been using separate 'extensions'
and 'capabilities' attributes there and 'version' is missing. Creating
a separate attribute for the trip allows us to reuse the assembly
form and verification.

Differential Revision: https://reviews.llvm.org/D75868