[Diag] Refactor diagnosticStringFor to return StringRef

[HassanElDesouky]

To make #33422 easier to review, I decided to open this PR.

When I was testing #33422 and returning diagnostic messages from the hash table, there was a mismatch in some diagnostic messages because there was an extra 0x01 byte at the end of every diagnostic message.

Currently, diagnosticStringFor returns a const char * obtained by calling data() on a StringRef coming from LocalizationProducer::getMessageOr. Since, StringRef isn't null-terminated, when converting it to a pointer and then back to a StringRef later - in DiagnosticInfo - causes the length to be wrong.

When changing diagnosticStringFor to return an llvm::StringRef instead and removing the calls to data(), the tests in #33422 will pass.

cc @xedin, @owenv

[xedin]

Makes sense, thank you!

[xedin]

@swift-ci please test