[clang-tidy][NFC] Fix buffer overflow in modernize-use-designated-initializers

Instance of DenseMap were copied into local variable,
and as a result reference to string stored in that local
variable were returned from function. At the end fix-it
were applied with already destroyed string causing some
non utf-8 characters to be printed.

Related to #80541

Author: PiotrZSL

clang-tools-extra/clang-tidy/modernize/UseDesignatedInitializersCheck.cpp

@@ -43,7 +43,7 @@ struct Designators {
   unsigned size() { return getCached().size(); }
 
   std::optional<llvm::StringRef> operator[](const SourceLocation &Location) {
-    const auto Designators = getCached();
+    const auto &Designators = getCached();
     const auto Result = Designators.find(Location);
     if (Result == Designators.end())
       return {};