Date/Time: Ensure that `get_feed_build_date()` correctly handles a modified...

Date/Time: Ensure that `get_feed_build_date()` correctly handles a modified post object with invalid date.

* Clarify in the documentation that the function returns `false` on failure.
* Consistently pass the return value through the `get_feed_build_date` filter.

Props Rarst, dd32, azaozz, tellyworth.
Fixes #48957.
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git-svn-id: https://core.svn.wordpress.org/trunk@46774 1a063a9b-81f0-0310-95a4-ce76da25c4cd