[ModuleSummary] use StringRefs to avoid a redundant copy; NFC

`Saver` is a StringSaver, which has a few overloads of `save` that all
ultimately just call `StringRef save(StringRef)`. Just take a StringRef
here instead of building up a std::string to convert it to a StringRef.

llvm-svn: 348650

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -4918,7 +4918,7 @@ void ModuleSummaryIndexBitcodeReader::setValueGUID(
   ValueIdToValueInfoMap[ValueID] = std::make_pair(
       TheIndex.getOrInsertValueInfo(
           ValueGUID,
-          UseStrtab ? ValueName : TheIndex.saveString(ValueName.str())),
+          UseStrtab ? ValueName : TheIndex.saveString(ValueName)),
       OriginalNameID);
 }