[analyzer] Fix body farm for Obj-C++ properties

When property is declared in a superclass (or in a protocol),
it still can be of CXXRecord type and Sema could've already
generated a body for us.  This patch joins two branches and
two ways of acquiring IVar in order to reuse the existing code.
And prevent us from generating l-value to r-value casts for
C++ types.

rdar://67416721

Differential Revision: https://reviews.llvm.org/D99194