[mlir][spirv] Migrate to use specalized enum attributes

Previously we are using IntegerAttr to back all SPIR-V enum
attributes. Therefore we all such attributes are showed like
IntegerAttr in IRs, which is barely readable and breaks
roundtripability of the IR. This commit changes to use
`EnumAttr` as the base directly so that we can have separate
attribute definitions and better IR printing.

Reviewed By: kuhar

Differential Revision: https://reviews.llvm.org/D131311