DependenciesScanner: include compiled module candidates for textual module interface in JSON output

Instead of replacing an interface file with its up-to-date compile module,
the dep-scanner should report potentially up-to-date module candidates either adjacent to
the interface file or in the prebuilt module cache. swift-driver should later pass down
these candidates to -compile-module-from-interface invocation and the front-end job
will check if one of the candidates is ready to use. The front-end job then either emits a forwarding
module to an up-to-date candidate or a binary module.

include/swift/Frontend/ModuleInterfaceLoader.h

@@ -237,8 +237,9 @@ public:
     bool SerializeDependencyHashes, bool TrackSystemDependencies,
     ModuleInterfaceLoaderOptions Opts);
 
-  std::string getUpToDateCompiledModuleForInterface(StringRef moduleName,
-                                                    StringRef interfacePath) override;
+  std::vector<std::string>
+  getCompiledModuleCandidatesForInterface(StringRef moduleName,
+                                          StringRef interfacePath) override;
 };
 
 struct InterfaceSubContextDelegateImpl: InterfaceSubContextDelegate {