Require every if/else path to return in AlwaysReturnInFilter

[GaryJones]

This follows on from #907 and fixes two remaining gaps in how AlwaysReturnInFilter decides whether a filter callback always returns a value.

The sniff only treated if as conditional, so a return in an else or elseif counted as one that always runs. A callback that returned only from its else, or from an if/elseif chain without an else, passed even though one path returned nothing:

add_filter( 'my_filter', function( $value ) { // Not flagged.
	if ( is_admin() ) {
		return 'admin';
	} elseif ( is_feed() ) {
		return 'feed';
	}
} );

The sniff now follows each path through if, elseif and else chains, written with braces, as else if, or with the alternative syntax. A chain only counts as returning when it ends in an else and every branch returns, and that applies to chains nested in a branch too. Other control structures, such as loops, switch and try, keep the previous leniency: a return anywhere inside them counts. jrfnl raised the wider version of this on #520, which also covers checking every path through those structures. This PR deals with if chains only.

The second commit fixes the check behind the terminating statement warning added for #719. An exit, die or throw anywhere in the callback's body turned the missing return error into that softer warning, even when it sat in a nested closure or arrow function, such as a shutdown action. The callback itself never terminates there, so this is now the error again.

Both changes can report errors on code that passed before: callbacks with a path that returns nothing, and callbacks whose only throw or exit is in a nested closure. That is worth a line in the changelog. I ran the old and new sniff over the 21 Automattic plugins I have locally and the results did not change, but this sniff only reported four issues across them, so that is a small sample.