Magic attribute values should get centralized

[MDoerner]

As discussed in review comments to PR #4641, we should centralize the magic attribute values like VB_UserMemId = -4 for enumerators in one place and always retrieve them from there.

A natural canditate for where the corresponding information can live is the TypeLib API that already defines corresponding enums.

[bclothier]

I also wonder if we need to provide aliases. For example, VB_UserMemId is just DispId. If we choose to create a new enum VbUserMemberId, its members should probably remap to existing WellKnownDispIds enum.

Another advantage of aliased enum would allow us to add values that are apparently known only to VB but not COM in general, an example being -520 which indicates its an Appearance member, whatever it means. This would help to keep the context straight at expense of extra complexity/maintenance.