[clang][cli] Add MarshallingInfoEnum multiclass

This patch introduces a tablegen multiclass called `MarshallingInfoEnum`. It has the same semantics as `MarshallingInfoString` had in combination with `AutoNormalizeEnum`, but it's easier to use and follows the convention used for other `MarshallingInfoXxx` multiclasses.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D97375