Use this to define the macro location rather than the "host" dir (which
is actually for the build and not the host). Furthermore, on Windows,
the build dir is /usr/lib/swift as the host content is in the SDK.
This prepares the tests for Windows.
Add a test case for Observable types that are extended from other
source files. Prior to the recent changes to make
`TypeRefinementContext` more lazy, this would trigger circular
references through the `TypeRefinementContextBuilder`.
Finishes rdar://112079160.
