Use htab_up in type copying

Use htab_up in type copying

This changes create_copied_types_hash to return an htab_up, then
modifies the callers to avoid explicit use of htab_delete.

gdb/ChangeLog
2020-09-17  Tom Tromey  <tom@tromey.com>

	* value.c (preserve_values): Update.
	* python/py-type.c (save_objfile_types): Update.
	* guile/scm-type.c (save_objfile_types): Update.
	* gdbtypes.h (create_copied_types_hash): Return htab_up.
	* gdbtypes.c (create_copied_types_hash): Return htab_up.
	* compile/compile-object-run.c (compile_object_run): Update.