[Explicit Module Build] Do not serialize binary .swiftmodule dependencies when building explicit interface dependencies

The build-system (swift-driver) is then responsible for validating all inputs to each module and re-scheduling affected modules.

Author: artemcm

lib/FrontendTool/FrontendTool.cpp

@@ -410,6 +410,13 @@ static bool buildModuleFromInterface(CompilerInstance &Instance) {
   bool IgnoreAdjacentModules = Instance.hasASTContext() &&
                                Instance.getASTContext().IgnoreAdjacentModules;
 
+  // When building explicit module dependencies, they are
+  // discovered by dependency scanner and the swiftmodule is already rebuilt
+  // ignoring candidate module. There is no need to serialized dependencies for
+  // validation purpose because the build system (swift-driver) is then
+  // responsible for checking whether inputs are up-to-date.
+  bool ShouldSerializeDeps = !FEOpts.ExplicitInterfaceBuild;
+
   // If an explicit interface build was requested, bypass the creation of a new
   // sub-instance from the interface which will build it in a separate thread,
   // and isntead directly use the current \c Instance for compilation.
@@ -422,8 +429,7 @@ static bool buildModuleFromInterface(CompilerInstance &Instance) {
     return ModuleInterfaceLoader::buildExplicitSwiftModuleFromSwiftInterface(
         Instance, Invocation.getClangModuleCachePath(),
         FEOpts.BackupModuleInterfaceDir, PrebuiltCachePath, ABIPath, InputPath,
-        Invocation.getOutputFilename(),
-        /* shouldSerializeDeps */ true,
+        Invocation.getOutputFilename(), ShouldSerializeDeps,
         Invocation.getSearchPathOptions().CandidateCompiledModules);
 
   return ModuleInterfaceLoader::buildSwiftModuleFromSwiftInterface(