[mlir][spirv] Allow specifying availability on enum attribute cases

Lots of SPIR-V ops take enum attributes and certain enum cases
need extra capabilities or extensions to be available. This commit
extends to allow specifying availability spec on enum cases.
Extra utility functions are generated for the corresponding enum
classes to return the availability requirement. The availability
interface implemention for a SPIR-V op now goes over all enum
attributes to collect the availability requirements.

Reviewed By: mravishankar

Differential Revision: https://reviews.llvm.org/D71947