Replace flag_strict_flex_arrays with DECL_NOT_FLEXARRAY in middle-end.

Replace flag_strict_flex_arrays with DECL_NOT_FLEXARRAY in middle-end.

We should not directly check flag_strict_flex_arrays in the
middle end. Instead, check DECL_NOT_FLEXARRAY(array_field_decl) which is set
by C/C++ FEs according to -fstrict-flex-arrays and the corresponding
attribute attached to the array_field.

As a result, We will lose the LEVEL information of -fstrict-flex-arrays in
the middle end. -Wstrict-flex-arrays will not be able to issue such
information. update the testing cases accordingly.

gcc/ChangeLog:

	* attribs.cc (strict_flex_array_level_of): Move this function to ...
	* attribs.h (strict_flex_array_level_of): Remove the declaration.
	* gimple-array-bounds.cc (array_bounds_checker::check_array_ref):
	replace the referece to strict_flex_array_level_of with
	DECL_NOT_FLEXARRAY.
	* tree.cc (component_ref_size): Likewise.

gcc/c/ChangeLog:

	* c-decl.cc (strict_flex_array_level_of): ... here.

gcc/testsuite/ChangeLog:

	* gcc.dg/Warray-bounds-flex-arrays-1.c: Delete the level information
	from the message issued by -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.
	* gcc.dg/Wstrict-flex-arrays-2.c: Likewise.
	* gcc.dg/Wstrict-flex-arrays-3.c: Likewise.
	* gcc.dg/Wstrict-flex-arrays.c: Likewise.