Use a SetVector when looking up the SPI attributes on imports

Using a SetVector fixes an issue where many source files imported the
same SPI group from the same module, the emitted private textual
interfaces superfluously repeated the `@_spi` attribute on the import.

rdar://problem/63681845

lib/Serialization/ModuleFile.h

@@ -816,8 +816,9 @@ public:
 
   /// Find all SPI names imported from \p importedModule by this module,
   /// collecting the identifiers in \p spiGroups.
-  void lookupImportedSPIGroups(const ModuleDecl *importedModule,
-                              SmallVectorImpl<Identifier> &spiGroups) const;
+  void lookupImportedSPIGroups(
+                         const ModuleDecl *importedModule,
+                         llvm::SmallSetVector<Identifier, 4> &spiGroups) const;
 
   /// Reports all link-time dependencies.
   void collectLinkLibraries(ModuleDecl::LinkLibraryCallback callback) const;