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/matcher.cpp

@@ -0,0 +1,39 @@
+// 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/matcher.h"
+
+namespace ct = ::clang::tooling;
+
+namespace Carbon {
+
+void Matcher::AddReplacement(const clang::SourceManager& sm,
+                             clang::CharSourceRange range,
+                             llvm::StringRef replacement_text) {
+  if (!range.isValid()) {
+    llvm::errs() << "Invalid range: " << range.getAsRange().printToString(sm)
+                 << "\n";
+    return;
+  }
+  if (sm.getDecomposedLoc(range.getBegin()).first !=
+      sm.getDecomposedLoc(range.getEnd()).first) {
+    llvm::errs() << "Range spans macro expansions: "
+                 << range.getAsRange().printToString(sm) << "\n";
+    return;
+  }
+  if (sm.getFileID(range.getBegin()) != sm.getFileID(range.getEnd())) {
+    llvm::errs() << "Range spans files: "
+                 << range.getAsRange().printToString(sm) << "\n";
+    return;
+  }
+
+  auto rep = ct::Replacement(sm, sm.getExpansionRange(range), replacement_text);
+  auto err = (*replacements)[std::string(rep.getFilePath())].add(rep);
+  if (err) {
+    llvm::report_fatal_error("Error with replacement `" + rep.toString() +
+                             "`: " + llvm::toString(std::move(err)) + "\n");
+  }
+}
+
+}  // namespace Carbon