New option -Wstrict-aliasing=2.

	* alias.c (alias_sets_might_conflict_p): New.
	* c-typeck.c (build_c_cast): Call it if warn_strict_aliasing > 1.
	* common.opt (Wstrict-aliasing=): New.
	* flags.h (warn_strict_aliasing): Change type to int.
	* opts.c (warn_strict_aliasing): Change type to int.
	(common_handle_option): Handle OPT_Wstrict_aliasing_.
	* tree.h (alias_sets_might_conflict_p): Declare it.
	* doc/invoke.tex (-Wstrict-aliasing=2): Document it.

From-SVN: r79222