swift234

[Clang importer] Wire up extensions for globals-as-members.

Wire up the extensions created when importing globals-as-members via
the same mechanisms we use for Objective-C categories, e.g.,
implementing loadAllExtensions() to find the extensions and lazily
loading their members via the member loader. Addresses most of my
comments on the way extensions were wired in.

Extend our testing for importing globals as members to two different
submodules, so we can see the separation of extensions.

As part of this, fold getOrCreateExtension into importDeclContextOf.