[OpenMP] Correctly diagnose conflicting target identifierers for AMDGPU

There are static checks on the target identifiers allowed in a single
TU. Previously theses checks were only applied to HIP even though they
should be the same for OpenMP targeting AMDGPU. Simply enable these
checks for OpenMP.

Reviewed By: JonChesterfield, yaxunl

Differential Revision: https://reviews.llvm.org/D152965