Move the language server into toolchain's busybox. (#4469)

Removes the separate language server binary; I'm not sure we need to
provide it. Instead, `carbon language-server` is added as a subcommand.

Moves //language_server to //toolchain/language_server. Splits into a
trivial language_server.h, and a substantive server.h. I wasn't sure
about a better name, but wanted the split similar to check/check.h,
lex/lex.h, etc. At the same time, the class is probably going to be a
little big so not a good fit to through into just a cpp file.

This fixes some style issues with the language server class, but
generally I'm trying to not address things here in order to keep it
simpler.

toolchain/driver/language_server_subcommand.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/language_server_subcommand.h"
+
+#include "toolchain/language_server/language_server.h"
+
+namespace Carbon {
+
+constexpr CommandLine::CommandInfo LanguageServerSubcommand::Info = {
+    .name = "language-server",
+    .help = R"""(
+Runs the language server.
+)""",
+};
+
+auto LanguageServerSubcommand::Run(DriverEnv& driver_env) -> DriverResult {
+  // TODO: Consider a way to override stdin, but it's a `FILE*` so less
+  // convenient to work with.
+  auto err = LanguageServer::Run(stdin, driver_env.output_stream);
+  if (!err.ok()) {
+    driver_env.error_stream << "error: " << err.error() << "\n";
+  }
+  return {.success = err.ok()};
+}
+
+}  // namespace Carbon