[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. This patch adds a
new lit substitution, %omp_powerpc_triple that, if the system is 32-bit or
64-bit, sets the powerpc triple accordingly. This was required to fix
some OpenMP tests that automatically populated the target architecture.

Reviewers: jdoerfert

Subscribers: cfe-commits guansong sstefan1 yaxunl delcypher

Tags: OpenMP clang LLVM

Differential Revision: https://reviews.llvm.org/D88594