[ms-cxxabi] Raise aggregate memptr alignment to 8 for x86_32

With this change, we give different results for __alignof than MSVC, but
our record layout is compatible.

Some data member pointers also now have a size that is not a multiple of
their alignment.

Fixes PR18618.

Reviewers: majnemer

Differential Revision: http://llvm-reviews.chandlerc.com/D2669