ignore -mconstructor-aliases when adding field paddings for asan

Summary:
When we are adding field paddings for asan even an empty dtor has to remain in the code,
so we ignore -mconstructor-aliases if the paddings are going to be added.

Test Plan: added a test

Reviewers: rsmith, rnk, rafael

Reviewed By: rafael

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D6038

llvm-svn: 220986