[analyzer] Make CoreEngine responsible for enqueueing Stmt Nodes.

Enqueue the nodes generated as the result of processing a statement
inside the Core Engine. This makes sure ExpEngine does not access
CoreEngine's private members and is more concise.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@143089 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed