[analyzer] Track class member initializer constructors path-sensitively.

The reasoning behind this change is similar to the previous commit, r334681.
Because members are already in scope when construction occurs, we are not
suffering from liveness problems, but we still want to figure out if the object
was constructed with construction context, because in this case we'll be able
to avoid trivial copy, which we don't always model perfectly. It'd also have
more importance when copy elision is implemented.

This also gets rid of the old CFG look-behind mechanism.

Differential Revision: https://reviews.llvm.org/D47350

llvm-svn: 334682