[MLIR] Add DefaultValuedEnumAttr decorator (#172916)

Introduce DefaultValuedEnumAttr, which similarly to DefaultValuedAttr
decorates an enum attribute to have a default value from a specific enum
case when not present. The default is constructed as the fully-qualified
enum case symbol.

In comparison to DefaultValuedAttr, this allows using a TableGen
EnumCase
variable instead of a raw string.

GitOrigin-RevId: 32243a54f82c64e4bef4deb357faf9044902fba3