[ParseableInterface] Add ‘forwarding modules’

A ‘forwarding module’ is a YAML file that’s meant to stand in for a .swiftmodule file and provide an up-to-date description of its dependencies, always using modification times.

When a ‘prebuilt module’ is first loaded, we verify that it’s up-to-date by hashing all of its dependencies. Since this is orders of magnitude slower than reading mtimes, we’ll install a `forwarding module` containing the mtimes of the now-validated dependencies.

lib/FrontendTool/FrontendTool.cpp

@@ -51,6 +51,7 @@
 #include "swift/Frontend/Frontend.h"
 #include "swift/Frontend/PrintingDiagnosticConsumer.h"
 #include "swift/Frontend/SerializedDiagnosticConsumer.h"
+#include "swift/Frontend/ParseableInterfaceModuleLoader.h"
 #include "swift/Frontend/ParseableInterfaceSupport.h"
 #include "swift/Immediate/Immediate.h"
 #include "swift/Index/IndexRecord.h"
@@ -585,7 +586,8 @@ static bool buildModuleFromParseableInterface(CompilerInvocation &Invocation,
   return ParseableInterfaceModuleLoader::buildSwiftModuleFromSwiftInterface(
       Instance.getASTContext(), Invocation.getClangModuleCachePath(),
       PrebuiltCachePath, Invocation.getModuleName(), InputPath,
-      Invocation.getOutputFilename());
+      Invocation.getOutputFilename(),
+      FEOpts.SerializeParseableModuleInterfaceDependencyHashes);
 }
 
 static bool compileLLVMIR(CompilerInvocation &Invocation,