[CFG] [analyzer] Add construction contexts for returning C++ objects in ObjC++.

Like any normal funciton, Objective-C message can return a C++ object
in Objective-C++. Such object would require a construction context.

This patch, therefore, is an extension of r327343 onto Objective-C++.

Differential Revision: https://reviews.llvm.org/D48608

llvm-svn: 338426