[analyzer][UninitializedObjectChecker] Refactoring p1.: ImmutableList factory is no longer static

This patch is the first part of a series of patches to refactor UninitializedObjectChecker. The goal of this effort is to

    Separate pointer chasing from the rest of the checker,
    Increase readability and reliability,
    Don't impact performance (too bad).

In this one, ImmutableList's factory is moved to FindUninitializedFields.

Differential Revision: https://reviews.llvm.org/D50503

llvm-svn: 339591