Log the location field before any backtrace

In PG13, we added the ability to add backtraces to the log output. 
After some practical experience with it, I think the order in which the 
BACKTRACE and the LOCATION fields are printed is wrong.  I propose we 
put the LOCATION field before the BACKTRACE field, not after.  This 
makes more sense because the location is effectively at the lowest level 
of the backtrace.

Patch attached.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services