Refactor WPDB::get_caller() into wp_debug_backtrace_summary() and improve the...

Refactor WPDB::get_caller() into wp_debug_backtrace_summary() and improve the functionality to provide enhanced context and a standardised default pretty format. Fixes #19589 


git-svn-id: https://core.svn.wordpress.org/trunk@19773 1a063a9b-81f0-0310-95a4-ce76da25c4cd