[OpenMP] Do not always emit unused extern variables

Currently, the precense of the OpenMP target declare metadata requires
that we always codegen a global declaration. This is undesirable in the
case that we could defer or omit this declaration as is common with
unused extern variables. This is important as it allows us, in the
runtime, to rely on static linking semantics to omit unused symbols so
they are not included when the user links it in.

This patch changes the check for always emitting these variables.
Because of this we also need to extend this logic to the generation of
the offloading entries. This has the result of derring the offload entry
generation to the canonical definitoin. So we are effectively assuming
whoever owns the storage for this variable will perform that operation.
This makes an exception for `link` attributes as those require their own
special handling.

Let me know if this is sound in the implementation, I do not have the
largest view of the standards here.

Fixes: https://github.com/llvm/llvm-project/issues/64133

Reviewed By: tianshilei1992

Differential Revision: https://reviews.llvm.org/D156368