PR c/83559 - -Wsuggest-attribute=const conflicts with -Wattributes warning about const attribute on function returning void

gcc/ChangeLog:

	PR c/83559
	* doc/extend.texi (attribute const): Fix a typo.
	* ipa-pure-const.c ((warn_function_const, warn_function_pure): Avoid
	issuing -Wsuggest-attribute for void functions.

gcc/testsuite/ChangeLog:

	PR c/83559
	* gcc.dg/const-2.c: New test.
	* gcc.dg/pure-3.c: New test.

From-SVN: r256188