Fix use-after-free in clang-apply-replacements

[benlangmuir]

Summary

Accidentally introduced a dangling StringRef in b4c6dc2e6637.

clang-tools-extra/clang-apply-replacements/lib/Tooling/ApplyReplacements.cpp

 #include "clang/Format/Format.h"
 #include "clang/Lex/Lexer.h"
 #include "clang/Rewrite/Core/Rewriter.h"
 #include "clang/Tooling/Core/Diagnostic.h"
 #include "clang/Tooling/DiagnosticsYaml.h"
 #include "clang/Tooling/ReplacementsYaml.h"
 #include "llvm/ADT/ArrayRef.h"
 #include "llvm/ADT/Optional.h"
+#include "llvm/ADT/StringSet.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/raw_ostream.h"
 
 using namespace llvm;
 using namespace clang;
 

 /// TranslationUnitDiagnostics.
 /// \param[in] SM Used to deduplicate paths.
 ///
 /// \returns A map mapping FileEntry to a set of Replacement targeting that
 /// file.
 static llvm::DenseMap<const FileEntry *, std::vector<tooling::Replacement>>
 groupReplacements(const TUReplacements &TUs, const TUDiagnostics &TUDs,
                   const clang::SourceManager &SM) {
-  std::set<StringRef> Warned;
+  llvm::StringSet<> Warned;
   llvm::DenseMap<const FileEntry *, std::vector<tooling::Replacement>>
       GroupedReplacements;
 
   // Deduplicate identical replacements in diagnostics unless they are from the
   // same TU.
   // FIXME: Find an efficient way to deduplicate on diagnostics level.
   llvm::DenseMap<const FileEntry *,
                  std::map<tooling::Replacement,