[lldb][NFCI] Constrain EventDataBytes creation (#79508)

There are 3 ways to create an EventDataBytes object: (const char *),
(llvm::StringRef), and (const void *, size_t len). All of these cases
can be handled under `llvm::StringRef`. Additionally, this allows us to
remove the otherwise unused `SetBytes`, `SwapBytes`, and
`SetBytesFromCString` methods.