[NFC] avoid AlignedCharArray in clang

[NFC] avoid AlignedCharArray in clang

As discussed in D65249, don't use AlignedCharArray or std::aligned_storage. Just use alignas(X) char Buf[Size];. This will allow me to remove AlignedCharArray entirely, and works on the current minimum version of Visual Studio.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@367274 91177308-0d34-0410-b5e6-96231b3b80d8