[clang] Avoid creating temporary instances of std::string (NFC) (#140988)

lookupTarget takes StringRef and internally creates an instance of
std::string with the StringRef as part of constructing Triple, so we
don't need to create temporary instances of std::string on our own.

Author: kazutakahirata

clang/lib/Driver/ToolChain.cpp

@@ -502,8 +502,7 @@ ToolChain::getTargetAndModeFromProgramName(StringRef PN) {
   StringRef Prefix(ProgName);
   Prefix = Prefix.slice(0, LastComponent);
   std::string IgnoredError;
-  bool IsRegistered =
-      llvm::TargetRegistry::lookupTarget(std::string(Prefix), IgnoredError);
+  bool IsRegistered = llvm::TargetRegistry::lookupTarget(Prefix, IgnoredError);
   return ParsedClangName{std::string(Prefix), ModeSuffix, DS->ModeFlag,
                          IsRegistered};
 }

clang/lib/Testing/CommandLineArgs.cpp

@@ -103,8 +103,7 @@ std::string getAnyTargetForTesting() {
     StringRef TargetName(Target.getName());
     if (TargetName == "x86-64")
       TargetName = "x86_64";
-    if (llvm::TargetRegistry::lookupTarget(std::string(TargetName), Error) ==
-        &Target) {
+    if (llvm::TargetRegistry::lookupTarget(TargetName, Error) == &Target) {
       return std::string(TargetName);
     }
   }