Refactor Matcher code to reduce per-Matcher boilerplate (#615)

This is in part to eliminate the "sm" and "lang_opts" variables that received comment, but also cuts back on the use of auto.

migrate_cpp/cpp_refactoring/matcher_manager.h

@@ -0,0 +1,58 @@
+// 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_MANAGER_H_
+#define MIGRATE_CPP_CPP_REFACTORING_MATCHER_MANAGER_H_
+
+#include "clang/ASTMatchers/ASTMatchFinder.h"
+#include "clang/Tooling/Core/Replacement.h"
+#include "migrate_cpp/cpp_refactoring/matcher.h"
+
+namespace Carbon {
+
+// Manages registration of AST matchers.
+class MatcherManager {
+ public:
+  explicit MatcherManager(Matcher::ReplacementMap* in_replacements)
+      : replacements(in_replacements) {}
+
+  // Registers Matcher implementations.
+  void Register(std::unique_ptr<MatcherFactory> factory) {
+    matchers.push_back(std::make_unique<MatchCallbackWrapper>(
+        &finder, std::move(factory), replacements));
+  }
+
+  auto GetFinder() -> clang::ast_matchers::MatchFinder* { return &finder; }
+
+ private:
+  // Adapts Matcher for use with MatchCallback.
+  class MatchCallbackWrapper
+      : public clang::ast_matchers::MatchFinder::MatchCallback {
+   public:
+    explicit MatchCallbackWrapper(clang::ast_matchers::MatchFinder* finder,
+                                  std::unique_ptr<MatcherFactory> in_factory,
+                                  Matcher::ReplacementMap* in_replacements)
+        : factory(std::move(in_factory)), replacements(in_replacements) {
+      finder->addMatcher(factory->GetAstMatcher(), this);
+    }
+
+    void run(const clang::ast_matchers::MatchFinder::MatchResult& match_result)
+        override {
+      factory->CreateMatcher(&match_result, replacements)->Run();
+    }
+
+   private:
+    std::unique_ptr<MatcherFactory> factory;
+    Matcher::ReplacementMap* const replacements;
+  };
+
+  Matcher::ReplacementMap* const replacements;
+  clang::ast_matchers::MatchFinder finder;
+  std::vector<std::unique_ptr<MatcherFactory>> factories;
+  std::vector<std::unique_ptr<MatchCallbackWrapper>> matchers;
+};
+
+}  // namespace Carbon
+
+#endif  // MIGRATE_CPP_CPP_REFACTORING_MATCHER_MANAGER_H_