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/base/llvm_tools.cpp

@@ -0,0 +1,41 @@
+// 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/base/llvm_tools.h"
+
+#include "common/command_line.h"
+
+// NOLINTBEGIN(readability-identifier-naming): External library name.
+#define CARBON_LLVM_MAIN_TOOL(Identifier, Name, BinName, MainFn)              \
+  extern auto MainFn(int argc, char** argv, const llvm::ToolContext& context) \
+      -> int;
+#include "toolchain/base/llvm_tools.def"
+// NOLINTEND(readability-identifier-naming): External library name.
+
+namespace Carbon {
+
+CARBON_DEFINE_ENUM_CLASS_NAMES(LLVMTool) = {
+#define CARBON_LLVM_TOOL(Identifier, Name, BinName, MainFn) Name,
+#include "toolchain/base/llvm_tools.def"
+};
+
+constexpr llvm::StringLiteral LLVMTool::BinNames[] = {
+#define CARBON_LLVM_TOOL(Identifier, Name, BinName, MainFn) BinName,
+#include "toolchain/base/llvm_tools.def"
+};
+
+constexpr LLVMTool::MainFnT* LLVMTool::MainFns[] = {
+#define CARBON_LLVM_TOOL(Identifier, Name, BinName, MainFn) &::MainFn,
+#include "toolchain/base/llvm_tools.def"
+};
+
+constexpr CommandLine::CommandInfo LLVMTool::SubcommandInfos[] = {
+#define CARBON_LLVM_TOOL(Identifier, Name, BinName, MainFn) \
+  {.name = Name,                                            \
+   .help = "Runs the LLVM " Name                            \
+           " command line tool with the provided arguments."},
+#include "toolchain/base/llvm_tools.def"
+};
+
+}  // namespace Carbon