Extract subcommand options from the driver file. (#4300)

I'm separating the options out so that it's easier to review. They
include a lot of boilerplate text that I think won't change much, and
makes it harder to review changes.

To explain filename differences, whereas `CodegenOptions` is shared (by
link and compile), `LinkOptions` and `CompileOptions` are
subcommand-specific. I'm planning to separate out the subcommands, so
I'm putting those in respective subcommand files. I'm still going to try
to use the `.h` to declare the interface, `.cpp` for bigger
implementation details (for better or worse, including comments on
options).

I'm also moving out corresponding Driver members to help shrink deltas
when refactoring. That is, the bodies aren't changing here, but a
refactoring of commands will make some changes. By moving the code to
different files now, it should be easier to identify what's changing
later.

---------

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

toolchain/driver/codegen_options.cpp

@@ -0,0 +1,28 @@
+// 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/codegen_options.h"
+
+namespace Carbon {
+
+void CodegenOptions::Build(CommandLine::CommandBuilder& b) {
+  b.AddStringOption(
+      {
+          .name = "target",
+          .help = R"""(
+Select a target platform. Uses the LLVM target syntax. Also known as a "triple"
+for historical reasons.
+
+This corresponds to the `target` flag to Clang and accepts the same strings
+documented there:
+https://clang.llvm.org/docs/CrossCompilation.html#target-triple
+)""",
+      },
+      [&](auto& arg_b) {
+        arg_b.Default(host);
+        arg_b.Set(&target);
+      });
+}
+
+}  // namespace Carbon