Sema: Reexport SPI via Swift exported imports but not clang exported imports

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

lib/AST/Module.cpp

@@ -3386,8 +3386,8 @@ void SourceFile::lookupImportedSPIGroups(
   for (auto &import : *Imports) {
     if (import.options.contains(ImportFlags::SPIAccessControl) &&
         (importedModule == import.module.importedModule ||
-         (imports.isImportedBy(importedModule, import.module.importedModule) &&
-          importedModule->isExportedAs(import.module.importedModule)))) {
+         imports.isImportedByViaSwiftOnly(importedModule,
+                                       import.module.importedModule))) {
       spiGroups.insert(import.spiGroups.begin(), import.spiGroups.end());
     }
   }