[analyzer] Fix crash when returning C++ objects from ObjC messages-to-nil.

the assertion is in fact incorrect: there is a cornercase in Objective-C++
in which a C++ object is not constructed with a constructor, but merely
zero-initialized. Namely, this happens when an Objective-C message is sent
to a nil and it is supposed to return a C++ object.

Differential Revision: https://reviews.llvm.org/D60988

llvm-svn: 359262