AST: Simplify CharUnits::alignmentAtOffset

CharUnits::alignmentAtOffset is equivalent to llvm::MinAlign but
slightly less efficient.  Use it's implementation instead.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202099 91177308-0d34-0410-b5e6-96231b3b80d8