Add ATTRIBUTE_UNUSED_RESULT to ref_ptr::release

Add ATTRIBUTE_UNUSED_RESULT to ref_ptr::release

This applies ATTRIBUTE_UNUSED_RESULT to ref_ptr::release and updates a
few spots to comply.  I believe one use in install_default_visualizer
was in error, fixed by this patch.

gdb/ChangeLog
2019-03-05  Tom Tromey  <tromey@adacore.com>

	* varobj.c (update_dynamic_varobj_children): Update.
	(install_default_visualizer): Use reset, not release.
	* value.c (set_internalvar): Update.
	* dwarf2loc.c (value_of_dwarf_reg_entry): Update.
	* common/gdb_ref_ptr.h (class ref_ptr) <release>: Add
	ATTRIBUTE_UNUSED_RESULT.