[NFC] [ClangImporter] Fully support lazy loading
Previously, the lazily-created extensions used for globals imported as members of a type used the lazy module loader, but `ClangImporter::Implementation::loadNamedMembers()` didn’t actually work for them. Other parts of the compiler instead contrived to avoid loading these members by name by forcing all members to load before any selective loading might occur.
This commit modifies that code path to accommodate extensions with no matching clang node, which is how these are represented. With this change, other parts of the compiler can unconditionally use the `LazyMemberLoader` whenever it is present.
There may be performance improvements from this change, but I don’t expect any functional changes.
