Widgets: Use a `foreach` loop instead of The Loop to iterate over posts in...

Widgets: Use a `foreach` loop instead of The Loop to iterate over posts in Recent Posts widget to avoid needing to reset a polluted global `$post`.

Props welcher, westonruter.
Amends [14607].
See #12320.
Fixes #37312.

Built from https://develop.svn.wordpress.org/trunk@41890


git-svn-id: https://core.svn.wordpress.org/trunk@41724 1a063a9b-81f0-0310-95a4-ce76da25c4cd