Use new/delete for do_module_cleanup

Use new/delete for do_module_cleanup

This changes do_module_cleanup to use new and delete.  It also removes
the use of the struct hack from this object -- this requires more
allocations for now, but this will be removed in a subsequent patch.

gdb/ChangeLog
2020-09-23  Tom Tromey  <tom@tromey.com>

	* compile/compile-object-run.c (struct do_module_cleanup): Add
	constructor, destructor.
	<objfile_name_string>: Don't use struct hack.
	(do_module_cleanup): Use delete.
	(compile_object_run): Use new.