Refactor Matcher code to reduce per-Matcher boilerplate (#615)

This is in part to eliminate the "sm" and "lang_opts" variables that received comment, but also cuts back on the use of auto.

migrate_cpp/cpp_refactoring/matcher_test_base.h

@@ -7,19 +7,57 @@
 
 #include "clang/ASTMatchers/ASTMatchFinder.h"
 #include "clang/Tooling/Core/Replacement.h"
+#include "clang/Tooling/Tooling.h"
 #include "gmock/gmock.h"
 #include "gtest/gtest.h"
+#include "migrate_cpp/cpp_refactoring/matcher_manager.h"
 
 namespace Carbon {
 
+// Matcher test framework.
+template <typename MatcherFactoryType>
 class MatcherTestBase : public ::testing::Test {
  protected:
+  MatcherTestBase() : matchers(&replacements) {
+    matchers.Register(std::make_unique<MatcherFactoryType>());
+  }
+
   // Expects that that the replacements produced by running the finder result in
   // the specified code transformation.
-  void ExpectReplacement(llvm::StringRef before, llvm::StringRef after);
+  void ExpectReplacement(llvm::StringRef before, llvm::StringRef after) {
+    auto factory =
+        clang::tooling::newFrontendActionFactory(matchers.GetFinder());
+    constexpr char Filename[] = "test.cc";
+    replacements.clear();
+    replacements.insert({Filename, {}});
+    ASSERT_TRUE(clang::tooling::runToolOnCodeWithArgs(
+        factory->create(), before, {}, Filename, "clang-tool",
+        std::make_shared<clang::PCHContainerOperations>(),
+        clang::tooling::FileContentMappings()));
+    EXPECT_THAT(replacements, testing::ElementsAre(testing::Key(Filename)));
+    llvm::Expected<std::string> actual =
+        clang::tooling::applyAllReplacements(before, replacements[Filename]);
 
-  std::map<std::string, clang::tooling::Replacements> replacements;
-  clang::ast_matchers::MatchFinder finder;
+    // Make a specific note if the matcher didn't make any changes.
+    std::string unchanged;
+    if (before == *actual) {
+      unchanged = "NOTE: Actual matches original text, no changes made.";
+    }
+
+    if (after.find('\n') == std::string::npos) {
+      EXPECT_THAT(*actual, testing::Eq(after.str())) << unchanged;
+    } else {
+      // Split lines to get gmock to get an easier-to-read error.
+      llvm::SmallVector<llvm::StringRef, 0> actual_lines;
+      llvm::SplitString(*actual, actual_lines, "\n");
+      llvm::SmallVector<llvm::StringRef, 0> after_lines;
+      llvm::SplitString(after, after_lines, "\n");
+      EXPECT_THAT(actual_lines, testing::ContainerEq(after_lines)) << unchanged;
+    }
+  }
+
+  Matcher::ReplacementMap replacements;
+  MatcherManager matchers;
 };
 
 }  // namespace Carbon