[mlir] Improve EnumProp, making it take an EnumInfo (#132349)

This commit improves the `EnumProp` class, causing it to wrap around an
`EnumInfo` just like` EnumAttr` does. This EnumProp also has logic for
converting to/from an integer attribute and for being read and written
as bitcode.

The following variants of `EnumProp` are provided:
- `EnumPropWithAttrForm` - an EnumProp that can be constructed from (and
will be converted to, if `storeInCustomAttribute` is true) a custom
attribute, like an `EnumAttr`, instead of a plain integer. This is meant
for backwards compatibility with code that uses enum attributes.

`NamedEnumProp` adds a "`mnemonic` `<` $enum `>`" syntax around the
enum, replicating a common pattern seen in MLIR printers and allowing
for reduced ambiguity.

`NamedEnumPropWithAttrForm` combines both of these extensions.

(Sadly, bytecode auto-upgrade is hampered by the lack of the ability to
optionally parse an attribute.)

Depends on #132148

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