[Dexter] Add DexDeclareAddress command and address function

This patch adds a new dexter command, DexDeclareAddress, which is used
to test the relative values of pointer variables. The motivation for
adding this command is to allow meaningful assertions to be made about
pointers that go beyond checking variable availability and null
equality.

The full explanation and syntax is in Commands.md.

Reviewed By: Orlando

Differential Revision: https://reviews.llvm.org/D111447