[Serialization] Support Swift only system module

Previously 'isSystemModule()' returns true only if the module is:
- Standard library
- Clang module and that is `IsSystem`
- Swift overlay for clang `IsSystem` module

Now:
- Clang module and that is `IsSystem`; or
- Swift overlay for clang `IsSystem` module
- Swift module found in either of these directories:
  - Runtime library directoris (including stdlib)
  - Frameworks in `-Fsystem` directories
  - Frameworks in `$SDKROOT/System/Library/Frameworks/` (Darwin)
  - Frameworks in `$SDKROOT/Library/Frameworks/` (Darwin)

rdar://problem/50516314

include/swift/Serialization/SerializedModuleLoader.h

@@ -51,7 +51,7 @@ protected:
   bool findModule(AccessPathElem moduleID,
                   std::unique_ptr<llvm::MemoryBuffer> *moduleBuffer,
                   std::unique_ptr<llvm::MemoryBuffer> *moduleDocBuffer,
-                  bool &isFramework);
+                  bool &isFramework, bool &isSystemModule);
 
   /// Attempts to search the provided directory for a loadable serialized
   /// .swiftmodule with the provided `ModuleFilename`. Subclasses must