MS ABI: Let non-virtual method overloads participate in vftable ordering

In the Microsoft ABI, the vftable is laid out as if all methods in every
overload set were declared in reverse order of declaration at the point
of declaration of the first overload in the set.

Previously we only considered virtual methods in an overload set, but
MSVC includes non-virtual methods for ordering purposes.

Fixes PR18902.

llvm-svn: 201722