[NFC] Return llvm::StringRef from StringExtractor::GetStringRef.

This patch removes the two variant of StringExtractor::GetStringRef that
return (non-)const references to std::string. The non-const one was
being abused to reinitialize the StringExtractor and its uses are
replaced by calls to the copy asignment operator. The const variant was
refactored to return an actual llvm::StringRef.

llvm-svn: 369493