[analyzer] Add a checker to manage dynamic type propagation.

[analyzer] Add a checker to manage dynamic type propagation.

Instead of sprinkling dynamic type info propagation throughout
ExprEngine, the added checker would add the more precise type
information on known APIs (Ex: ObjC alloc, new) and propagate
the type info in other cases (ex: ObjC init method, casts (the second is
not implemented yet)).

Add handling of ObjC alloc, new and init to the checker.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@161357 91177308-0d34-0410-b5e6-96231b3b80d8