[ExplicitModuleBuild] Support -vfsoverlay swift option

Support `-vfsoverlay` swift option for explicit module build (including
caching build). Previously, if the interface file is discovered from a
location that is remapped by overlay, module cannot be built correctly.
Make sure the overlay options are passed down to all interface
compilaiton command.

For caching build, need to make sure the overlay itself is part of the CAS
file system so the downstream compilation can discover that.

rdar://123655183

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -1879,6 +1879,13 @@ InterfaceSubContextDelegateImpl::InterfaceSubContextDelegateImpl(
   genericSubInvocation.getSearchPathOptions().ExplicitSwiftModuleMap =
     explicitSwiftModuleMap.str();
 
+  // Pass down VFSOverlay flags (do not need to inherit the options because
+  // FileSystem is shared).
+  for (auto &Overlay : searchPathOpts.VFSOverlayFiles) {
+    GenericArgs.push_back("-vfsoverlay");
+    GenericArgs.push_back(Overlay);
+  }
+
   // Load plugin libraries for macro expression as default arguments
   genericSubInvocation.getSearchPathOptions().PluginSearchOpts =
       searchPathOpts.PluginSearchOpts;