Comments: Add a `$post_id` parameter to `get_comments_number_text()` and `comments_number()`.

This allow for using these functions outside of the loop.

Props donmhico, garrett-eclipse, Hinjiriyo, imath.
Fixes #48772.
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git-svn-id: https://core.svn.wordpress.org/trunk@47076 1a063a9b-81f0-0310-95a4-ce76da25c4cd