fix -fsanitize-address-field-padding for the cases with virtual base classes

Summary: Correctly compute the non-virtual size of a class.

Test Plan: Build SPEC 2016 with -fsanitize-address-field-padding

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D5848

llvm-svn: 220089