[MS] Increase default new alignment for win64 and test it

Summary:
This raises __STDCPP_DEFAULT_NEW_ALIGNMENT__ from 8 to 16 on Win64.
This matches platforms that follow the usual `2 * sizeof(void*)`
alignment requirement for malloc. We might want to consider making that
the default rather than relying on long double alignment.

Fixes PR35356

Reviewers: STL_MSFT, rsmith

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D40277

llvm-svn: 318723