[TableGen] Generating enum definitions and utility functions

Enum attributes can be defined using `EnumAttr`, which requires all its cases
to be defined with `EnumAttrCase`. To facilitate the interaction between
`EnumAttr`s and their C++ consumers, add a new EnumsGen TableGen backend
to generate a few common utilities, including an enum class, `llvm::DenseMapInfo`
for the enum class, conversion functions from/to strings.

This is controlled via the `-gen-enum-decls` and `-gen-enum-defs` command-line
options of `mlir-tblgen`.

PiperOrigin-RevId: 252209623