Teach SwiftModules To Embed Incremental Information

Take advantage of the binary swiftdeps serialization utliities built during #32131. Add a new optional information block to swiftdeps files. For now, don't actually serialize swiftdeps information.

Frontends will use this information to determine whether to write incremental dependencies across modules into their swiftdeps files. We will then teach the driver to deserialize the data from this section and integrate it into its incremental decision making.

lib/AST/FrontendSourceFileDepGraphFactory.cpp

@@ -251,7 +251,7 @@ bool fine_grained_dependencies::emitReferenceDependencies(
                              SF, outputPath, depTracker, alsoEmitDotFile)
                               .construct();
 
-  bool hadError = writeFineGrainedDependencyGraph(diags, outputPath, g);
+  bool hadError = writeFineGrainedDependencyGraphToPath(diags, outputPath, g);
 
   // If path is stdout, cannot read it back, so check for "-"
   assert(outputPath == "-" || g.verifyReadsWhatIsWritten(outputPath));