Add subcommands and busybox entry points for LLVM tools (#5049)

This adds support for most of the remaining LLVM command line tools
using a generic, generated wrapper. The subcommand interface for these
is (much) less interesting than our other subcommands, but it gives us
a uniform and consistent layer.

Note that I structured these as nested sub-sub-commands below an `llvm`
subcommand because of an expectation that we will want to add more, and
ones that don't use this generic layer. Some concrete future work:

- Add the `opt` and `llc` tools as subcommands for easier debugging and
  experimentation with LLVM IR output from Carbon's toolchain.
- Potentially sink `lld` below the `llvm` layer given that it has
  significantly less user visibility than commands like `clang`.

Unfortunately, the current driver subcommand APIs make nested
subcommands awkward. I've added a somewhat rough hack here to let the
LLVM tools go in, but there are some TODOs that I want to address in
a follow-up that works to adjust the structure of this code to be more
conducive to nesting like this.

Depends on #5048 -- only the last commit should be reviewed here.

---------

Co-authored-by: Geoff Romer <gromer@google.com>

toolchain/driver/llvm_subcommand.cpp

@@ -0,0 +1,73 @@
+// 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/llvm_subcommand.h"
+
+#include "common/command_line.h"
+#include "llvm/TargetParser/Host.h"
+#include "llvm/TargetParser/Triple.h"
+#include "toolchain/base/llvm_tools.h"
+#include "toolchain/driver/llvm_runner.h"
+
+namespace Carbon {
+
+static constexpr CommandLine::CommandInfo SubcommandInfo = {
+    .name = "llvm",
+    .help = R"""(
+Runs LLVM's command line tools with the provided arguments.
+
+This subcommand provides access to a collection of LLVM's command line tools via
+further subcommands. For each of these tools, their command line can be provided
+using positional arguments.
+)""",
+};
+
+auto LLVMOptions::Build(CommandLine::CommandBuilder& b,
+                        DriverSubcommand* subcommand,
+                        DriverSubcommand** selected_subcommand) -> void {
+  // Add further subcommands for each LLVM tool.
+  for (LLVMTool tool : LLVMTool::Tools) {
+    // TODO: The subcommand info for each tool is weirdly stored in the
+    // `LLVMTool` class instead of here where it makes more logical sense.
+    // Either we should figure out how to move it to here or we should more
+    // fully document the oddity of having it in the `LLVMTool` class.
+    //
+    // TODO: Currently, the command line subsystem's help isn't as user friendly
+    // for the generated subcommands below this as it could be. Because each of
+    // these has a completely generic and stamped out info, the `help` output is
+    // very repetitive and doesn't actually contribute much information.
+    b.AddSubcommand(tool.subcommand_info(), [&](auto& sub_b) {
+      sub_b.AddStringPositionalArg(
+          {
+              .name = "ARG",
+              .help = R"""(
+Arguments passed to the LLVM tool.
+)""",
+          },
+          [&](auto& arg_b) { arg_b.Append(&args); });
+      sub_b.Do([=, this] {
+        subcommand_tool = tool;
+        *selected_subcommand = subcommand;
+      });
+    });
+  }
+
+  // The `llvm` subcommand can't be used directly.
+  b.RequiresSubcommand();
+}
+
+LLVMSubcommand::LLVMSubcommand() : DriverSubcommand(SubcommandInfo) {}
+
+auto LLVMSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
+  LLVMRunner runner(driver_env.installation, driver_env.vlog_stream);
+
+  // Don't run arbitrary LLVM tools and libraries when fuzzing.
+  if (!DisableFuzzingExternalLibraries(driver_env, "llvm")) {
+    return {.success = false};
+  }
+
+  return {.success = runner.Run(options_.subcommand_tool, options_.args)};
+}
+
+}  // namespace Carbon