Member lookup macro expansion performance 5.9

[DougGregor]

• Explanation: When determining which type definitions and extensions need to have macros expanded to satisfy a member name lookup request, the name lookup code is iterating over all of the extensions of a given type and their members. When there are many extensions or members (e.g., thousands), the individually-cheap checks here cause significant compile-time performance problems. Memoize the member checks on a per-extension basis, and keep a separate (much smaller) list of those extensions that have any interesting macro behavior, so we don't need to walk the full list. On a source file containing 6,000 extensions of the same type, reduces compile time by ~25%, returning it to Swift 5.8 levels.
• Scope: Moderate. Member name lookup is a fundamental operation used everywhere in the compiler, so the code changes will be exercised everywhere. The change to that code is fairly small and macro-centric.
• Risk: Low; the code changed here only does any meaningful work when macros are present on the types or extensions, so it's unlikely to break anything not using macros even if it were wrong.
• Issue: rdar://109543968
• Original pull request: Member lookup macro expansion performance #66257

[DougGregor]

@swift-ci please test