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/driver.cpp

@@ -10,12 +10,9 @@
 
 #include "common/command_line.h"
 #include "common/version.h"
-#include "llvm/ADT/ArrayRef.h"
-#include "llvm/ADT/StringExtras.h"
-#include "llvm/ADT/StringRef.h"
-#include "llvm/IR/LLVMContext.h"
-#include "llvm/Support/Path.h"
-#include "toolchain/base/value_store.h"
+#include "toolchain/driver/clang_subcommand.h"
+#include "toolchain/driver/compile_subcommand.h"
+#include "toolchain/driver/link_subcommand.h"
 
 namespace Carbon {
 
@@ -27,6 +24,7 @@ struct Options {
 
   bool verbose;
 
+  ClangSubcommand clang;
   CompileSubcommand compile;
   LinkSubcommand link;
 
@@ -63,6 +61,11 @@ auto Options::Build(CommandLine::CommandBuilder& b) -> void {
       },
       [&](CommandLine::FlagBuilder& arg_b) { arg_b.Set(&verbose); });
 
+  b.AddSubcommand(ClangOptions::Info, [&](CommandLine::CommandBuilder& sub_b) {
+    clang.BuildOptions(sub_b);
+    sub_b.Do([&] { subcommand = &clang; });
+  });
+
   b.AddSubcommand(CompileOptions::Info,
                   [&](CommandLine::CommandBuilder& sub_b) {
                     compile.BuildOptions(sub_b);