[Clang][OpenMP][OMPIRBuilder] Fix getTargetEntryUniqueInfo to fatal error...

[Clang][OpenMP][OMPIRBuilder] Fix getTargetEntryUniqueInfo to fatal error instead of assert when it cannot generate a unique ID

This prevents access of uninitialized ID components later, or continuing progress
further making errors harder to track down.

Hoepfully this will be the last in my long series of mistakes in recent
commits relating to this area of code!