[libcxxabi] Reorder base class initializers in libc++abi tests to prevent -Wreorder

Summary:
This patch fixes -Wreorder warnings on test classes with virtual bases. Since the compiler is performing the reordering anyway this change *should* have NFC.

However the test notes that it is checking that "virtual base classes work properly". Since initialization order is clearly part of correctness I want to confirm that this wasn't an intentional mistake.



Reviewers: mclow.lists, howard.hinnant

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D21396

llvm-svn: 272821