[spirv] Use I32EnumAttr for enum attributes

[spirv] Use I32EnumAttr for enum attributes

This saves us the excessive string conversions and comparisons in
verification and transformation and scopes them only to parsing
and printing, which are meant for I/O so string conversions should
be fine.

In order to do this, changed the custom assembly format of
spv.module regarding addressing model and memory model.

PiperOrigin-RevId: 256149856