Move BitEnumAttr from SPIRVBase.td to OpBase.td

BitEnumAttr is a mechanism for modelling attributes whose value is
a bitfield. It should not be scoped to the SPIR-V dialect and can
be used by other dialects too.

This CL is mostly shuffling code around and adding tests and docs.
Functionality changes are:

* Fixed to use `getZExtValue()` instead of `getSExtValue()` when
  getting the value from the underlying IntegerAttr for a case.
* Changed to auto-detect whether there is a case whose value is
  all bits unset (i.e., zero). If so handle it specially in all
  helper methods.

PiperOrigin-RevId: 277964926