MS ABI: Don't ICE for pointers to pointers to members of incomplete classes

CodeGen would try to come up with an LLVM IR type for a pointer to
member type on the way to forming an LLVM IR type for a pointer to
pointer to member type.

However, if the pointer to member representation has not been locked in yet,
we would not be able to come up with a pointer to member IR type.

In these cases, make the pointer to member type an incomplete type.
This will make the pointer to pointer to member type a pointer to an
incomplete type.  If the class eventually obtains an inheritance model,
we will make the pointer to member type represent the actual inheritance
model.

Differential Revision: http://reviews.llvm.org/D5373

llvm-svn: 218084