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/matcher_manager.h

@@ -34,7 +34,7 @@ class MatcherManager {
                                   std::unique_ptr<MatcherFactory> in_factory,
                                   Matcher::ReplacementMap* in_replacements)
         : factory(std::move(in_factory)), replacements(in_replacements) {
-      finder->addMatcher(factory->GetAstMatcher(), this);
+      factory->AddMatcher(finder, this);
     }
 
     void run(const clang::ast_matchers::MatchFinder::MatchResult& match_result)