ModuleInterface: pass-down explicit Swift module paths when building module from interface

Author: nkcsgexi

include/swift/Frontend/ModuleInterfaceLoader.h

@@ -131,10 +131,12 @@ struct ModuleInterfaceLoaderOptions {
   bool remarkOnRebuildFromInterface = false;
   bool disableInterfaceLock = false;
   bool disableImplicitSwiftModule = false;
+  std::vector<std::string> explicitSwiftModules;
   ModuleInterfaceLoaderOptions(const FrontendOptions &Opts):
     remarkOnRebuildFromInterface(Opts.RemarkOnRebuildFromModuleInterface),
     disableInterfaceLock(Opts.DisableInterfaceFileLock),
-    disableImplicitSwiftModule(Opts.DisableImplicitModules) {}
+    disableImplicitSwiftModule(Opts.DisableImplicitModules),
+    explicitSwiftModules(Opts.ExplicitSwiftModules) {}
   ModuleInterfaceLoaderOptions() = default;
 };
 /// A ModuleLoader that runs a subordinate \c CompilerInvocation and

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -1201,6 +1201,14 @@ InterfaceSubContextDelegateImpl::InterfaceSubContextDelegateImpl(
     subInvocation.getFrontendOptions().DisableImplicitModules = true;
     GenericArgs.push_back("-disable-implicit-swift-modules");
   }
+  subInvocation.getFrontendOptions().ExplicitSwiftModules =
+    LoaderOpts.explicitSwiftModules;
+  // Dependencies scanner shouldn't know any explict Swift modules to use.
+  // Adding these argumnets may not be necessary.
+  // FIXME: remove it?
+  for (auto EM: LoaderOpts.explicitSwiftModules) {
+    GenericArgs.push_back(ArgSaver.save((llvm::Twine("-swift-module-file=") + EM).str()));
+  }
   if (clangImporter) {
     // We need to add these extra clang flags because explict module building
     // related flags are all there: -fno-implicit-modules, -fmodule-map-file=,