IRGen: Simplify alignment calculation in setBitFieldInfo

Take advantage of CharUnits::alignmentAtOffset instead of calculating it
by hand.

Differential Revision: http://llvm-reviews.chandlerc.com/D2862


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202098 91177308-0d34-0410-b5e6-96231b3b80d8