Add an implicit dtor CFG node just before C++ 'delete' expressions.

This paves the way for adding support for modeling the destructor of a
region before it is deleted. The statement "delete <expr>" now generates
this series of CFG elements:

  1. <expr>
  2. [B1.1]->~Foo() (Implicit destructor)
  3. delete [B1.1]

Patch by Karthik Bhat!