This is a new attempt at a reexport feature for SPI decls. The previous
behavior was to reexport SPIs only between modules with both `@_exported` and
an export-as relationship. The limitation on export-as turned out to be too
restrictive in some cases.
We may be tempted to reexport SPIs through all exported imports. However,
the exported imports are very common between clang module and it can lead
to surprises if dependencies between clang modules end up exporting SPIs from
unexpected modules.
As a middle ground, reexport SPI only through Swift `@_exported` dependencies,
and not through clang reexports. While this is a new distinction between Swift
and clang dependencies, I believe it provides the expected behavior and
the result is more straightforward than the current logic.
rdar://115901208
@_exported exports SPIs only when the exported module defines export_as
pointing to the exporter module. Other reexports do not reexport SPIs.
This is to prevent SPI reexporting to get out of hands with the wide
reexports of the Objective-C world.
rdar://102335473
Enable transitive imports of all SPI groups through @_exported imports.
This brings to SPI the same behavior that we have for API.
```
// Module A
@_spi(S) public func foo() {}
// Module B
@_exported import A
// Module C
@_spi(S) import B
foo() // SPI imported through the reexport of A from B
```
rdar://101566534
