[CodeCompletion] Protect against a couple of null Types
Fix two crashes related to unresolved-member completion where either the
EnumDecl itself is missing, or its elements have not been type-checked.
Incidentally, resolve the type of the enum elements in the case where I
have observed this happening.
rdar://problem/26860249 (master)
rdar://problem/27083795 (Swift 2.3)
Conflicts:
test/IDE/complete_unresolved_members.swift
