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/matcher.h

@@ -0,0 +1,35 @@
+// 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_MATCHER_H_
+#define MIGRATE_CPP_CPP_REFACTORING_MATCHER_H_
+
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/Tooling/Core/Replacement.h"
+
+namespace Carbon {
+
+// This is an abstract class with helpers to make it easier to write matchers.
+class Matcher : public clang::ast_matchers::MatchFinder::MatchCallback {
+ public:
+  // Alias these to elide the namespaces in subclass headers.
+  using MatchFinder = clang::ast_matchers::MatchFinder;
+  using Replacements = clang::tooling::Replacements;
+
+  explicit Matcher(std::map<std::string, Replacements>& in_replacements)
+      : replacements(&in_replacements) {}
+
+ protected:
+  // Replaces the given range with the specified text.
+  void AddReplacement(const clang::SourceManager& sm,
+                      clang::CharSourceRange range,
+                      llvm::StringRef replacement_text);
+
+ private:
+  std::map<std::string, Replacements>* const replacements;
+};
+
+}  // namespace Carbon
+
+#endif  // MIGRATE_CPP_CPP_REFACTORING_MATCHER_H_