Move free_all_objfiles to program_space

Move free_all_objfiles to program_space

This changes free_all_objfiles to be a method on program_space, in
line with the other changes to treat program_space as a container for
objfiles.

gdb/ChangeLog
2019-12-12  Tom Tromey  <tom@tromey.com>

	* symfile.c (symbol_file_clear): Update.
	* progspace.h (struct program_space) <free_all_objfiles>: Declare
	method.
	* progspace.c (program_space::free_all_objfiles): New method.
	* objfiles.h (free_all_objfiles): Don't declare.
	* objfiles.c (free_all_objfiles): Move to program_space.

Change-Id: I908b549d2981b6005f7ca181fc0e6d24fc8b7b6f