Provide counted_by attribute to flexible array member field

'counted_by (COUNT)'
     The 'counted_by' attribute may be attached to the C99 flexible
     array member of a structure.  It indicates that the number of the
     elements of the array is given by the field "COUNT" in the
     same structure as the flexible array member.
     GCC may use this information to improve detection of object size information
     for such structures and provide better results in compile-time diagnostics
     and runtime features like the array bound sanitizer and
     the '__builtin_dynamic_object_size'.

     For instance, the following code:

          struct P {
            size_t count;
            char other;
            char array[] __attribute__ ((counted_by (count)));
          } *p;

     specifies that the 'array' is a flexible array member whose number
     of elements is given by the field 'count' in the same structure.

     The field that represents the number of the elements should have an
     integer type.  Otherwise, the compiler reports an error and
     ignores the attribute.

     When the field that represents the number of the elements is assigned a
     negative integer value, the compiler treats the value as zero.

     An explicit 'counted_by' annotation defines a relationship between
     two objects, 'p->array' and 'p->count', and there are the following
     requirementthat on the relationship between this pair:

        * 'p->count' must be initialized before the first reference to
          'p->array';

        * 'p->array' has _at least_ 'p->count' number of elements
          available all the time.  This relationship must hold even
          after any of these related objects are updated during the
          program.

     It's the user's responsibility to make sure the above requirements
     to be kept all the time.  Otherwise the compiler reports
     warnings, at the same time, the results of the array bound
     sanitizer and the '__builtin_dynamic_object_size' is undefined.

     One important feature of the attribute is, a reference to the
     flexible array member field uses the latest value assigned to
     the field that represents the number of the elements before that
     reference.  For example,

            p->count = val1;
            p->array[20] = 0;  // ref1 to p->array
            p->count = val2;
            p->array[30] = 0;  // ref2 to p->array

     in the above, 'ref1' uses 'val1' as the number of the elements
     in 'p->array', and 'ref2' uses 'val2' as the number of elements
     in 'p->array'.

gcc/c-family/ChangeLog:

	* c-attribs.cc (handle_counted_by_attribute): New function.
	(attribute_takes_identifier_p): Add counted_by attribute to the list.
	* c-common.cc (c_flexible_array_member_type_p): ...To this.
	* c-common.h (c_flexible_array_member_type_p): New prototype.

gcc/c/ChangeLog:

	* c-decl.cc (flexible_array_member_type_p): Renamed and moved to...
	(add_flexible_array_elts_to_size): Use renamed function.
	(is_flexible_array_member_p): Use renamed function.
	(verify_counted_by_attribute): New function.
	(finish_struct): Use renamed function and verify counted_by
	attribute.
	* c-tree.h (lookup_field): New prototype.
	* c-typeck.cc (lookup_field): Expose as extern function.
	(tagged_types_tu_compatible_p): Check counted_by attribute for
	structure type.

gcc/ChangeLog:

	* doc/extend.texi: Document attribute counted_by.

gcc/testsuite/ChangeLog:

	* gcc.dg/flex-array-counted-by.c: New test.
	* gcc.dg/flex-array-counted-by-7.c: New test.
	* gcc.dg/flex-array-counted-by-8.c: New test.

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