Lift the 10-type limit for AlignedCharArrayUnion

This patch uses C++11 parameter packs and constexpr functions
to allow AlignedCharArrayUnion to hold an arbitrary number of
types.

Differential Revision: https://reviews.llvm.org/D28429


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@291503 91177308-0d34-0410-b5e6-96231b3b80d8