[analyzer] Make sure a temporary object region matches its initial bindings.

When creating a temporary region (say, when a struct rvalue is used as
the base of a member expr), make sure we account for any derived-to-base
casts. We don't actually record these in the LazyCompoundVal that
represents the rvalue, but we need to make sure that the temporary region
we're creating (a) matches the bindings, and (b) matches its expression.

Most of the time this will do exactly the same thing as before, but it
fixes spurious "garbage value" warnings introduced in r175234 by the use
of lazy bindings to model trivial copy constructors.

<rdar://problem/13265460>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175830 91177308-0d34-0410-b5e6-96231b3b80d8