Keep track of those types and extensions that have any macro expansions,
so that when we need to check whether to expand macros to satisfy name
lookup, we only look through that list---rather than looking at every
extension again.
This improves compile times significantly for a source file with a
large number of extensions on a single type. However, there is still
much to do here.
Keep track of which types/extensions have members that could be produced by
a macro expansion, including the names of those members. Use this to
avoid walking into extensions or type definitions to expand macros
when they don't have any related macros.
