[MS ABI] Correctly mangling vbase destructors

They are a little bit of a special case in the mangling. They are always
mangled without taking into account their virtual-ness of the
destructor. They are also mangled to return void, unlike the actual
destructor.

This fixes PR31931.

Differential Revision: https://reviews.llvm.org/D29912

llvm-svn: 295010