Issue 4957: parser.yy: loc_on_music -> loc_on_copy

Issue 4957: parser.yy: loc_on_music -> loc_on_copy

The function loc_on_music has been renamed to loc_on_copy and has been
changed to create a copy for a number of different expression types.
This makes \xxx.yyy behave similar to \xxx-yyy with regard to copying
the original expression in most cases, and it also helps with default
arguments of music/void/event/scheme functions of more types than just
music.