ClangImporter: permit redundancy in the Generate PCM job

`-Xcc` flags are internal to the C compiler; we have `-vfsoverlay` now
which directly passes the VFS overlay. Because the VFS overlay flags are
automatically passed along to the clang scanner, we need to pass along
`-vfsoverlay` to the Swift frontend to ensure that any VFS overlays are
honoured. Most likely this was a workaround for `-vfsoverlay` not
getting propagated originally and when the restriction was lifted the
exercised codepath was not heavily tested.

Author: compnerd

lib/ClangImporter/ClangModuleDependencyScanner.cpp

@@ -143,8 +143,6 @@ ModuleDependencyVector ClangImporter::bridgeClangModuleDependencies(
     if (!ctx.CASOpts.EnableCaching) {
       auto &overlayFiles = invocation.getMutHeaderSearchOpts().VFSOverlayFiles;
       for (auto overlay : overlayFiles) {
-        if (llvm::is_contained(ctx.SearchPathOpts.VFSOverlayFiles, overlay))
-          continue;
         swiftArgs.push_back("-vfsoverlay");
         swiftArgs.push_back(overlay);
       }