[Caching] Remove capture clang extra files

After removing the CASFS implementation for clang modules, there is no
need to capture clang extra file that sets up the VFS for the clang
modules since all content imported by ClangImporter is dependency
scanned and available via include-tree. This saves more ClangImporter
instance when caching is enabled.

Update the test to check that clang content found via `-Xcc` VFS options
can currently work without capture the headermaps and vfs overlays.

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -595,14 +595,6 @@ ModuleDependencyScanner::getMainModuleDependencyInfo(ModuleDecl *mainModule) {
   auto mainDependencies = ModuleDependencyInfo::forSwiftSourceModule(
        {}, buildCommands, {}, {}, {});
 
-  if (ScanASTContext.CASOpts.EnableCaching) {
-    std::vector<std::string> clangDependencyFiles;
-    clangImporter->addClangInvovcationDependencies(clangDependencyFiles);
-    llvm::for_each(clangDependencyFiles, [&](std::string &file) {
-      mainDependencies.addAuxiliaryFile(file);
-    });
-  }
-
   llvm::StringSet<> alreadyAddedModules;
   // Compute Implicit dependencies of the main module
   {