Win64: Pass member pointers larger than 8 bytes by reference

The Win64 ABI docs on MSDN say that arguments bigger than 8 bytes are
passed by reference.  Prior to this change, we were only applying this
logic to RecordType arguments.  This affects both the Itanium and
Microsoft C++ ABIs.

Reviewers: majnemer

Differential Revision: http://reviews.llvm.org/D3587