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/Frontend/ModuleInterfaceSupport.cpp

@@ -130,7 +130,7 @@ static void printImports(raw_ostream &out,
 
     // SPI attribute on imports
     if (Opts.PrintSPIs) {
-      SmallVector<Identifier, 4> spis;
+      llvm::SmallSetVector<Identifier, 4> spis;
       M->lookupImportedSPIGroups(importedModule, spis);
       for (auto spiName : spis)
         out << "@_spi(" << spiName << ") ";