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

include/swift/Serialization/SerializedModuleLoader.h

@@ -365,8 +365,9 @@ public:
          SmallVectorImpl<AbstractFunctionDecl *> &results) const override;
 
   virtual void
-  lookupImportedSPIGroups(const ModuleDecl *importedModule,
-                         SmallVectorImpl<Identifier> &spiGroups) const override;
+  lookupImportedSPIGroups(
+                const ModuleDecl *importedModule,
+                llvm::SmallSetVector<Identifier, 4> &spiGroups) const override;
 
   Optional<CommentInfo> getCommentForDecl(const Decl *D) const override;