log: Prefix log messages with function name if -logsourcelocations is set

Summary:

Prefix log messages with function name if -logfunctionnames is set, exactly like `-logthreadnames but for function names instead of thread names.

This is a small developer ergonomics improvement: I've found this to be a cheap/simple way to correlate log output and originating function.

This is a backport of core#19809

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10951