IRGen: Simplify alignment calculation in setBitFieldInfo
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: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@202098 91177308-0d34-0410-b5e6-96231b3b80d8

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