[analyzer][UninitializedObjectChecker] Refactoring p2.: Moving pointer chasing to a separate file

In this patch, the following classes and functions have been moved to a header file:

    FieldChainInfo
    FindUninitializedFields
    isPrimitiveType

This also meant that they moved from anonymous namespace to clang::ento.

Code related to pointer chasing now relies in its own file.

There's absolutely no functional change in this patch -- its literally just copy pasting.

Differential Revision: https://reviews.llvm.org/D50504

llvm-svn: 339595