Avoid conversion to std::function of cc1_main.

toolchain/driver/clang_runner.cpp

@@ -146,8 +146,7 @@ auto ClangRunner::Run(llvm::ArrayRef<llvm::StringRef> args) -> bool {
   // Note the subprocessing will effectively call `clang -cc1`, which turns into
   // `carbon-busybox clang -cc1`, which results in an equivalent `clang_main`
   // call.
-  std::function<int(llvm::SmallVectorImpl<const char*> & cc1_args)> cc1_main =
-      [](llvm::SmallVectorImpl<const char*>& cc1_args) -> int {
+  auto cc1_main = [](llvm::SmallVectorImpl<const char*>& cc1_args) -> int {
     // cc1_args[0] will be the `clang_path` so we don't need the prepend arg.
     llvm::ToolContext tool_context = {
         .Path = cc1_args[0], .PrependArg = "clang", .NeedsPrependArg = false};