Add a new warning option -Wstrict-flex-arrays.

Add a new warning option -Wstrict-flex-arrays.

'-Wstrict-flex-arrays'
     Warn about inproper usages of flexible array members according to
     the LEVEL of the 'strict_flex_array (LEVEL)' attribute attached to
     the trailing array field of a structure if it's available,
     otherwise according to the LEVEL of the option
     '-fstrict-flex-arrays=LEVEL'.

     This option is effective only when LEVEL is bigger than 0.
     Otherwise, it will be ignored with a warning.

     when LEVEL=1, warnings will be issued for a trailing array
     reference of a structure that have 2 or more elements if the
     trailing array is referenced as a flexible array member.

     when LEVEL=2, in addition to LEVEL=1, additional warnings will be
     issued for a trailing one-element array reference of a structure if
     the array is referenced as a flexible array member.

     when LEVEL=3, in addition to LEVEL=2, additional warnings will be
     issued for a trailing zero-length array reference of a structure if
     the array is referenced as a flexible array member.

gcc/ChangeLog:

	* doc/invoke.texi: Document -Wstrict-flex-arrays option.
	* gimple-array-bounds.cc (check_out_of_bounds_and_warn): Add two more
	arguments.
	(array_bounds_checker::check_array_ref): Issue warnings for
	-Wstrict-flex-arrays.
	* opts.cc (finish_options): Issue warning for unsupported combination
	of -Wstrict_flex_arrays and -fstrict-flex-array.
	* tree-vrp.cc (execute_ranger_vrp): Enable the pass when
	warn_strict_flex_array is true.

gcc/c-family/ChangeLog:

	* c.opt (Wstrict-flex-arrays): New option.

gcc/testsuite/ChangeLog:

	* gcc.dg/Warray-bounds-flex-arrays-1.c: Update testing case with
	-Wstrict-flex-arrays.
	* gcc.dg/Warray-bounds-flex-arrays-2.c: Likewise.
	* gcc.dg/Warray-bounds-flex-arrays-3.c: Likewise.
	* gcc.dg/Warray-bounds-flex-arrays-4.c: Likewise.
	* gcc.dg/Warray-bounds-flex-arrays-5.c: Likewise.
	* gcc.dg/Warray-bounds-flex-arrays-6.c: Likewise.
	* c-c++-common/Wstrict-flex-arrays.c: New test.
	* gcc.dg/Wstrict-flex-arrays-2.c: New test.
	* gcc.dg/Wstrict-flex-arrays-3.c: New test.
	* gcc.dg/Wstrict-flex-arrays.c: New test.