[spirv] Add support for BitEnumAttr

Certain enum classes in SPIR-V, like function/loop control and memory
access, are bitmasks. This CL introduces a BitEnumAttr to properly
model this and drive auto-generation of verification code and utility
functions. We still store the attribute using an 32-bit IntegerAttr
for minimal memory footprint and easy (de)serialization. But utility
conversion functions are adjusted to inspect each bit and generate
"|"-concatenated strings for the bits; vice versa.

Each such enum class has a "None" case that means no bit is set. We
need special handling for "None". Because of this, the logic is not
general anymore. So right now the definition is placed in the SPIR-V
dialect. If later this turns out to be useful for other dialects,
then we can see how to properly adjust it and move to OpBase.td.

Added tests for SPV_MemoryAccess to check and demonstrate.

PiperOrigin-RevId: 269350620