Fix MSVC build of AlignedCharArrayUnion

Use constexpr recursion for alignof like we do for sizeof. Seems to work
with Clang and MSVC. Also, don't recurse twice to avoid slowdowns in
compilers that don't memoize constexpr results (Clang).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291514 91177308-0d34-0410-b5e6-96231b3b80d8