Exclude debug log events marked "private" from MWDebug
Respect the "private" context variable for debug log events when passing
events from MediaWiki\Logger\LegacyLogger to MWDebug::debugMsg. Passing
debug log events marked as private to MWDebug was a regression
introduced by the PSR-3 logging system.
Restore handling of $dest argument to wfDebug which was removed in 1.25
with the PSR-3 logging conversion. The documentation for $dest = 'log'
has also been removed. This third debug log event handling option was
not implemented in the PSR-3 logging conversion in 1.25. A follow up
change will remove known usage of $dest = 'log' in core.
Bug: T122644
Change-Id: Ib1d999b8b54e584e3944b46e9163a700f11c2e72