dumpcpp: Fix Clang warnings

- Use const ref to avoid copies of variables
- Use range-based
- Streamline code a bit; loop over maps directly
  instead of key lists
- Avoid repetitive calls to toLatin1()
- Signedness issue converting QVariant::Type

Change-Id: I04ec73548f0a56edea8e3c0e8ee76c23a65aa07b
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>