Fix alignment of class derived from empty class.

The record layout code didn't properly take into account that
an empty class at offset 0 can have an alignment greater than 1.

Patch by Andrea Di Biagio.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@186370 91177308-0d34-0410-b5e6-96231b3b80d8