Add skeletal format subcommand. (#4383)

This extracts out the SourceBuffer handling of `-` in order to trivially
share it.

Note this still has a number of TODOs, it's just setting up the
essential subcommand infrastructure, with some tests demonstrating that
it at least does something.

toolchain/format/format.cpp

@@ -0,0 +1,36 @@
+// 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/format/format.h"
+
+#include "toolchain/lex/token_index.h"
+#include "toolchain/lex/tokenized_buffer.h"
+
+namespace Carbon::Format {
+
+// TODO: Add support for formatting line ranges (will need flags too).
+auto Format(const Lex::TokenizedBuffer& tokens, llvm::raw_ostream& out)
+    -> bool {
+  if (tokens.has_errors()) {
+    // TODO: Error recovery.
+    return false;
+  }
+  llvm::ListSeparator sep(" ");
+  for (auto token : tokens.tokens()) {
+    switch (tokens.GetKind(token)) {
+      case Lex::TokenKind::FileStart:
+        break;
+      case Lex::TokenKind::FileEnd:
+        out << "\n";
+        break;
+      default:
+        // TODO: More dependent formatting.
+        out << sep << tokens.GetTokenText(token);
+        break;
+    }
+  }
+  return true;
+}
+
+}  // namespace Carbon::Format