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/fn_inserter.cpp

@@ -0,0 +1,36 @@
+// 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/fn_inserter.h"
+
+#include "clang/ASTMatchers/ASTMatchers.h"
+
+namespace cam = ::clang::ast_matchers;
+
+namespace Carbon {
+
+FnInserter::FnInserter(std::map<std::string, Replacements>& in_replacements,
+                       cam::MatchFinder* finder)
+    : Matcher(in_replacements) {
+  // TODO: Switch from isExpansionInMainFile to isDefinition. That should then
+  // include `for (const auto* redecl : func->redecls())` to generate
+  // replacements.
+  finder->addMatcher(
+      cam::functionDecl(cam::isExpansionInMainFile(), cam::hasTrailingReturn())
+          .bind(Label),
+      this);
+}
+
+void FnInserter::run(const cam::MatchFinder::MatchResult& result) {
+  const auto* decl = result.Nodes.getNodeAs<clang::FunctionDecl>(Label);
+  if (!decl) {
+    llvm::report_fatal_error(std::string("getNodeAs failed for ") + Label);
+  }
+  auto begin = decl->getBeginLoc();
+  // Replace the first token in the range, `auto`.
+  auto range = clang::CharSourceRange::getTokenRange(begin, begin);
+  AddReplacement(*(result.SourceManager), range, "fn");
+}
+
+}  // namespace Carbon