Expand RelativeMemberInfo to LegacyRelativeMemberInfo | MinimallyEncodedRelativeMemberInfo
Summary:
Part of https://linear.app/comm/issue/ENG-6366/rename-minimallyencodedrawthreadinfo-to-rawthreadinfo
Just like the rest of the stack we're going to expand RelativeMemberInfo and remove the Legacy* variant wherever possible.
Depends on D10725
Test Plan: flow, this was simple IDE "Inline alias..." refactor
Reviewers: ashoat, ginsu, tomek
Reviewed By: ashoat
Differential Revision: https://phab.comm.dev/D10726