[Dependency Scanning] Add ability for -scan-dependencies action to serialize and deserialize dependency scanner cache from a .moddepcache file.

Using the serialization format added in https://github.com/apple/swift/pull/37585.

- Add load/save code for the `-scan-dependencies` code-path.
- Add `libSwiftDriver` entry-points to load/store the cache of a given scanner instance.

include/swift/DependencyScan/DependencyScanningTool.h

@@ -57,6 +57,14 @@ public:
                   const std::vector<BatchScanInput> &BatchInput,
                   const llvm::StringSet<> &PlaceholderModules);
 
+  /// Writes the current `SharedCache` instance to a specified FileSystem path.
+  void serializeCache(llvm::StringRef path);
+  /// Loads an instance of a `ModuleDependenciesCache` to serve as the `SharedCache`
+  /// from a specified FileSystem path.
+  bool loadCache(llvm::StringRef path);
+  /// Discard the tool's current `SharedCache` and start anew.
+  void resetCache();
+
 private:
   /// Using the specified invocation command, instantiate a CompilerInstance
   /// that will be used for this scan.