Make the objfile constructor private

Make the objfile constructor private

This changes the objfile constructor to be private, changing the
callers to use a factory method.  This isn't perhaps strictly needed
for the goal of this series -- changing the container model of
objfiles -- but is a nice symmetry.

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

	* symfile.c (symbol_file_add_with_addrs): Use objfile::make.
	* objfiles.h (struct objfile): Make constructor private.
	<make>: New static method.
	* jit.c (jit_object_close_impl): Update.

Change-Id: I42e07bc80a88cf3322ace94ffe869ae5788bcb29