[OpenMP] Add Error Handling for Conflicting Pointer Sizes for Target Offload

Summary:
This patch adds an error to Clang that detects if OpenMP offloading is used
between two architectures with incompatible pointer sizes. This ensures that
the data mapping can be done correctly and solves an issue in code generation
generating the wrong size pointer.

Reviewer: jdoerfert

Subscribers:

Tags: #OpenMP #Clang

Differential Revision: