[analyzer] pr37270: Track constructor target region, even if just a variable.

The very idea of construction context implies that first the object is
constructed, and then later, in a separate moment of time, the constructed
object goes into scope, i.e. becomes "live".

Most construction contexts require path-sensitive tracking of the constructed
object region in order to compute the outer expressions accordingly before
the object becomes live.

Semantics of simple variable construction contexts don't immediately require
that such tracking happens in path-sensitive manner, but shortcomings of the
analyzer force us to track it path-sensitively as well. Namely, whether
construction context was available at all during construction is a
path-sensitive information. Additionally, path-sensitive tracking takes care of
our liveness problems that kick in as the temporal gap between construction and
going-into-scope becomes larger (eg., due to copy elision).

Differential Revision: https://reviews.llvm.org/D47305

llvm-svn: 334681