For-range in replacement logic (#631)

This replaces GetAstMatcher with AddMatcher because cxxForRangeStmt is a StatementMatcher. addMatcher has multiple definitions (https://clang.llvm.org/doxygen/classclang_1_1ast__matchers_1_1MatchFinder.html) and so this approach allows using the right addMatcher without writing per-call overloads.

To handle the `var`, I'm considering something like moving VarDecl logic into a VarMatcherBase so that I can just use CXXForRangeStmt's getLoopVariable. The problem is a for-range statement has multiple VarDecls, and getLoopVariable may be the easiest way to identify the real one.

migrate_cpp/cpp_refactoring/for_range.h

@@ -0,0 +1,31 @@
+// 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_FOR_RANGE_H_
+#define MIGRATE_CPP_CPP_REFACTORING_FOR_RANGE_H_
+
+#include "migrate_cpp/cpp_refactoring/matcher.h"
+
+namespace Carbon {
+
+// Updates variable declarations for `var name: Type`.
+class ForRange : public Matcher {
+ public:
+  using Matcher::Matcher;
+  void Run() override;
+
+ private:
+  auto GetTypeStr(const clang::VarDecl& decl) -> std::string;
+};
+
+class ForRangeFactory : public MatcherFactoryBase<ForRange> {
+ public:
+  void AddMatcher(
+      clang::ast_matchers::MatchFinder* finder,
+      clang::ast_matchers::MatchFinder::MatchCallback* callback) override;
+};
+
+}  // namespace Carbon
+
+#endif  // MIGRATE_CPP_CPP_REFACTORING_FOR_RANGE_H_