Fix two assertion failures related to invalid UTF-8

[ahoppen]

• Fix an incorrect assertion in Cursor.swift
  • This assertion returned false if we reached the end of the file, while it should return true.
• Make SourceLocationConverter handle invalid UTF-8
  • The SourceLocationConverter previously worked on String, which is lossy if the source contains invalid UTF-8. Make it work on SyntaxText and RawTriviaPiece so it’s source accurate.

While at it, also fix a few things in swift-parser-cli:

• Show diagnostics from operator folding when running swift-parser-cli print-diags
• Run the diagnostic formatter during round-trip testing
  • This allows us to reduce crashes in the diagnostic formatter.
• Output reduced test case as bytes instead of a string
  • This allows us to reduce invalid UTF-8, which were lossily converted when creating a String.

[ahoppen]

@swift-ci Please test