Debugger permanently disabled after trying unsupported bitness

1. Have a 64 bit build of Creator without qtcreatorcdbext64.
2. Open a project in this Creator.
3. Create a build configuration using a 32-bit Qt and one using a 64-bit Qt.
4. Make the 64 bit build active.
5. Try debugging.
   This works.
6. Make the 32 bit build active.
7. Try debugging.
   This does not work because of QTCREATORBUG-18248.
8. Make the 64 bit build active again.
9. Try debugging.
   This does not work anymore because debugging is disabled completely.

When Creator is not able to debug a particular build, debugging other builds should still be possible.