Do not insert asan paddings after fields that have flexible arrays.

Summary:
We should avoid a tail padding not only if the last field
has zero size but also if the last field is a struct with a flexible array.

If/when http://reviews.llvm.org/D5478 is committed,
this will also handle the case of structs with zero-sized arrays.

Reviewers: majnemer, rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5924