Apply performance-unnecessary-value-param to clang-tidy.

With minor manual tweaks. No functionality change intended.

llvm-svn: 272795
diff --git a/clang-tools-extra/clang-tidy/tool/ClangTidyMain.cpp b/clang-tools-extra/clang-tidy/tool/ClangTidyMain.cpp
index 2662241..d295917 100644
--- a/clang-tools-extra/clang-tidy/tool/ClangTidyMain.cpp
+++ b/clang-tools-extra/clang-tidy/tool/ClangTidyMain.cpp
@@ -338,7 +338,7 @@
       return 1;
     }
     llvm::outs() << "Enabled checks:";
-    for (auto CheckName : EnabledChecks)
+    for (const auto &CheckName : EnabledChecks)
       llvm::outs() << "\n    " << CheckName;
     llvm::outs() << "\n\n";
     return 0;