[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.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@348650 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed