[Sema] Restrict reexported SPIs to modules with an export_as relationship

@_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

lib/Serialization/SerializedModuleLoader.cpp

@@ -1494,7 +1494,8 @@ void SerializedASTFile::lookupImportedSPIGroups(
       continue;
 
     if (dep.Import->importedModule == importedModule ||
-        imports.isImportedBy(importedModule, dep.Import->importedModule)) {
+        (imports.isImportedBy(importedModule, dep.Import->importedModule) &&
+         importedModule->isExportedAs(dep.Import->importedModule))) {
       spiGroups.insert(dep.spiGroups.begin(), dep.spiGroups.end());
     }
   }