Make llvm::StringRef to std::string conversions explicit.

This is how it should've been and brings it more in line with
std::string_view. There should be no functional change here.

This is mostly mechanical from a custom clang-tidy check, with a lot of
manual fixups. It uncovers a lot of minor inefficiencies.

This doesn't actually modify StringRef yet, I'll do that in a follow-up.

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -859,7 +859,7 @@ BitcodeReader::BitcodeReader(BitstreamCursor Stream, StringRef Strtab,
                              LLVMContext &Context)
     : BitcodeReaderBase(std::move(Stream), Strtab), Context(Context),
       ValueList(Context, Stream.SizeInBytes()) {
-  this->ProducerIdentification = ProducerIdentification;
+  this->ProducerIdentification = std::string(ProducerIdentification);
 }
 
 Error BitcodeReader::materializeForwardReferencedFunctions() {