[analyzer][UninitializedObjectChecker] Refactoring p4.: Wrap FieldRegions and...

[analyzer][UninitializedObjectChecker] Refactoring p4.: Wrap FieldRegions and reduce weight on FieldChainInfo

Before this patch, FieldChainInfo used a spaghetti: it took care of way too many cases,
even though it was always meant as a lightweight wrapper around
ImmutableList<const FieldRegion *>.
This problem is solved by introducing a lightweight polymorphic wrapper around const
FieldRegion *, FieldNode. It is an interface that abstracts away special cases like
pointers/references, objects that need to be casted to another type for a proper note
messages.

Changes to FieldChainInfo:

  * Now wraps ImmutableList<const FieldNode &>.
  * Any pointer/reference related fields and methods were removed
  * Got a new add method. This replaces it's former constructors as a way to create a
    new FieldChainInfo objects with a new element.

Changes to FindUninitializedField:

  * In order not to deal with dynamic memory management, when an uninitialized field is
    found, the note message for it is constructed and is stored instead of a
    FieldChainInfo object. (see doc around addFieldToUninits).

Some of the test files are changed too, from now on uninitialized pointees of references
always print "uninitialized pointee" instead of "uninitialized field" (which should've
really been like this from the beginning).

I also updated every comment according to these changes.

Differential Revision: https://reviews.llvm.org/D50506

llvm-svn: 339599