add get_comments_popup_link() function

[amereservant]

While recently writing a theme, I came across the need to get the output from comments_popup_link() as a string only to find there isn't a WP function that does this.

Looking at the code in comment_template.php, it looks like it'd be very simple to add and since most of the functions like it do have a get_* version of them, I figured I'd make the suggestion. Thanks.

Previously proposed in #17763, which also has a patch.