Add a special RecordLayoutBuilder for the Microsoft C++ ABI.

All it does right now is add space for two vtable pointers instead of one
when a class has both virtual methods and virtual bases. This is a requirement
of the Microsoft ABI, since it has separate vtables for methods and bases. Other
stuff will come up over time, but we'll cross those bridges when we get to
them.

llvm-svn: 111493