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.cpp

@@ -0,0 +1,29 @@
+// 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 "migrate_cpp/cpp_refactoring/for_range.h"
+
+#include "clang/ASTMatchers/ASTMatchers.h"
+
+namespace cam = ::clang::ast_matchers;
+
+namespace Carbon {
+
+static constexpr char Label[] = "ForRange";
+
+void ForRange::Run() {
+  const auto& stmt = GetNodeAsOrDie<clang::CXXForRangeStmt>(Label);
+
+  // Wrap `in` with spaces so that `for (auto i:items)` has valid results.
+  AddReplacement(clang::CharSourceRange::getTokenRange(stmt.getColonLoc(),
+                                                       stmt.getColonLoc()),
+                 " in ");
+}
+
+void ForRangeFactory::AddMatcher(cam::MatchFinder* finder,
+                                 cam::MatchFinder::MatchCallback* callback) {
+  finder->addMatcher(cam::cxxForRangeStmt().bind(Label), callback);
+}
+
+}  // namespace Carbon