ms_struct layout replaces platform-specific behavior like

useBitFieldTypeAlignment() and appears to ignore the special
bit-packing semantics of __attribute__((packed)).

Further flesh out an already-extensive comment.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@201282 91177308-0d34-0410-b5e6-96231b3b80d8