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_subcommand.cpp

@@ -0,0 +1,50 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/driver/clang_subcommand.h"
+
+#include "llvm/TargetParser/Host.h"
+#include "toolchain/driver/clang_runner.h"
+
+namespace Carbon {
+
+constexpr CommandLine::CommandInfo ClangOptions::Info = {
+    .name = "clang",
+    .help = R"""(
+Runs Clang on arguments.
+
+This is equivalent to running the `clang` command line directly, and provides
+the full command line interface.
+
+Use `carbon clang -- ARGS` to pass flags to `clang`. Although there are
+currently no flags for `carbon clang`, the `--` reserves the ability to add
+flags in the future.
+
+This is provided to help guarantee consistent compilation of C++ files, both
+when Clang is invoked directly and when a Carbon file importing a C++ file
+results in an indirect Clang invocation.
+)""",
+};
+
+auto ClangOptions::Build(CommandLine::CommandBuilder& b) -> void {
+  b.AddStringPositionalArg(
+      {
+          .name = "ARG",
+          .help = R"""(
+Arguments passed to Clang.
+)""",
+      },
+      [&](auto& arg_b) { arg_b.Append(&args); });
+}
+
+// TODO: This lacks a lot of features from the main driver code. We may need to
+// add more.
+// https://github.com/llvm/llvm-project/blob/main/clang/tools/driver/driver.cpp
+auto ClangSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
+  std::string target = llvm::sys::getDefaultTargetTriple();
+  ClangRunner runner(driver_env.installation, target, driver_env.vlog_stream);
+  return {.success = runner.Run(options_.args)};
+}
+
+}  // namespace Carbon