Exclude debug log events marked "private" from MWDebug

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