Add a clang subcommand. (#4322)

This makes something like `bazel run :toolchain -- clang -- -c test.cpp`
work, because that can be run in-process. Note that `bazel run
:toolchain -- clang -- test.cpp` still requires subprocessing, and does
not work.

Note, the vision here is that we are trying to align how clang and
carbon compile c++ code. This is work towards intertwining command
execution.

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/driver/clang_runner_test.cpp

@@ -152,5 +152,47 @@ TEST(ClangRunnerTest, LinkCommandEcho) {
   EXPECT_THAT(out, StrEq(""));
 }
 
+TEST(ClangRunnerTest, NoArgs) {
+  const auto install_paths =
+      InstallPaths::MakeForBazelRunfiles(Testing::GetExePath());
+  std::string verbose_out;
+  llvm::raw_string_ostream verbose_os(verbose_out);
+  std::string target = llvm::sys::getDefaultTargetTriple();
+  ClangRunner runner(&install_paths, target, &verbose_os);
+  std::string out;
+  std::string err;
+  EXPECT_FALSE(RunWithCapturedOutput(out, err, [&] { return runner.Run({}); }))
+      << "Verbose output from runner:\n"
+      << verbose_out << "\n";
+
+  EXPECT_THAT(out, StrEq(""));
+  EXPECT_THAT(err, HasSubstr("error: no input files"));
+}
+
+TEST(ClangRunnerTest, DashC) {
+  std::filesystem::path test_file =
+      WriteTestFile("test.cpp", "int test() { return 0; }");
+
+  const auto install_paths =
+      InstallPaths::MakeForBazelRunfiles(Testing::GetExePath());
+  std::string verbose_out;
+  llvm::raw_string_ostream verbose_os(verbose_out);
+  std::string target = llvm::sys::getDefaultTargetTriple();
+  ClangRunner runner(&install_paths, target, &verbose_os);
+  std::string out;
+  std::string err;
+  EXPECT_TRUE(
+      RunWithCapturedOutput(out, err,
+                            [&] {
+                              return runner.Run({"-c", test_file.string()});
+                            }))
+      << "Verbose output from runner:\n"
+      << verbose_out << "\n";
+
+  // No output should be produced.
+  EXPECT_THAT(out, StrEq(""));
+  EXPECT_THAT(err, StrEq(""));
+}
+
 }  // namespace
 }  // namespace Carbon