the CPU backend uses

As a side-effect, this also fixes all current memory leaks in
the CPU backend

Bob/static compiler

CMake: Avoid exporting symbols from static archives that the

NGraph DSO links to privately

With this change, the unit tests need to link against LLVM and Clang
directly since NGraph no longer exports those symbols.

cleanup headers used by generated code

CPU: Remove codegen directory if it exists

Bob/codewriter4

Adding Resnet50 Readme to private-ngraph-cpp as requested in PR#2282 …