Separated out the Onone module handling from stdlib handling

Also refactored the Onone module predicate. I believe there is no functional change, but I don't understand the correctness of what was there.

include/swift/Frontend/Frontend.h

@@ -449,7 +449,7 @@ public:
   void freeContextAndSIL();
 
 private:
-  bool loadStdlibAndMaybeSwiftOnoneSupport();
+  bool loadStdlib();
   ModuleDecl *importUnderlyingModule();
   ModuleDecl *importBridgingHeader();