Dependencies Scanner: report compiled Swift module paths if they are available

For the explicit module mode, swift-driver uses -compile-module-from-interface to
generate modules from interfaces found by the dependency scanner. However, we don't
need to build the binary module if up-to-date modules are available, either adjacent
to the interface file or in the prebuilt module cache directory. This patch teaches
dependencies scanner to report these ready-to-use binary modules.

include/swift/Serialization/SerializedModuleLoader.h

@@ -200,6 +200,11 @@ public:
   virtual Optional<ModuleDependencies> getModuleDependencies(
       StringRef moduleName, ModuleDependenciesCache &cache,
       InterfaceSubContextDelegate &delegate) override;
+
+  virtual std::string getUpToDateCompiledModuleForInterface(StringRef moduleName,
+                                                      StringRef interfacePath) {
+    return std::string();
+  }
 };
 
 /// Imports serialized Swift modules into an ASTContext.