[libcxx] [test] Silence warning C4324 for MSVC.

This warning "structure was padded due to alignment specifier" says
that the compiler is going to do exactly what you asked it to do.
It's triggered by the tests for over-aligned dynamic memory allocation.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@314257 91177308-0d34-0410-b5e6-96231b3b80d8