[PATCH] c/67882 - improve -Warray-bounds for invalid offsetof

[Martin Sebor]

Gcc attempts to diagnose invalid offsetof expressions whose member
designator is an array element with an out-of-bounds index. The
logic in the function that does this detection is incomplete, leading
to false negatives. Since the result of the expression in these cases
can be surprising, this patch tightens up the logic to diagnose more
such cases.