Add warn_unused_result attribute for memory-related functions in libiberty.

2019-06-10  Martin Liska  <mliska@suse.cz>

	* ansidecl.h (ATTRIBUTE_WARN_UNUSED_RESULT): New macro.
	* libiberty.h (xmalloc): Use it.
	(xrealloc): Likewise.
	(xcalloc): Likewise.
	(xstrdup): Likewise.
	(xstrndup): Likewise.
	(xmemdup): Likewise.

From-SVN: r272110