Add tests to the cpp_refactoring tool. (#539)

This refactors the main.cpp out into a file structure that should make it easier to add more matchers/tests.

migrate_cpp/cpp_refactoring/fn_inserter.h

@@ -0,0 +1,26 @@
+// 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
+
+#ifndef MIGRATE_CPP_CPP_REFACTORING_FN_INSERTER_H_
+#define MIGRATE_CPP_CPP_REFACTORING_FN_INSERTER_H_
+
+#include "migrate_cpp/cpp_refactoring/matcher.h"
+
+namespace Carbon {
+
+// Inserts `fn` for functions and methods.
+class FnInserter : public Matcher {
+ public:
+  explicit FnInserter(std::map<std::string, Replacements>& in_replacements,
+                      MatchFinder* finder);
+
+  void run(const MatchFinder::MatchResult& result) override;
+
+ private:
+  static constexpr char Label[] = "FnInserter";
+};
+
+}  // namespace Carbon
+
+#endif  // MIGRATE_CPP_CPP_REFACTORING_FN_INSERTER_H_