[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.

lib/DependencyScan/ModuleDependencyCacheSerialization.cpp

@@ -472,7 +472,7 @@ bool swift::dependencies::module_dependency_cache_serialization::
       "loading inter-module dependency graph", path);
   auto buffer = llvm::MemoryBuffer::getFile(path);
   if (!buffer)
-    return false;
+    return true;
 
   return readInterModuleDependenciesCache(*buffer.get(), cache);
 }