swift234

[test] We expect to find NSString in the Foundation module.

Import our fake Foundation rather than redeclaring NSString in SILGen's
fake "gizmo" module...then fix up other tests to more carefully use SILGen's
mock SDK in avoiding the real set of overlays...using -enable-source-import.

In the mid-term SILGen should probably switch to using the shared
clang-importer-sdk in test/Inputs/. In the long term we need that to be
using proper modules too rather than -enable-source-import.

No intended functionality change, but without this the next commit breaks
things (because members are deserialized less eagerly).

Swift SVN r27195