[OpenMP] Delay more diagnostics of potentially non-emitted code
Even code in target and declare target regions might not be emitted.
With this patch we delay more diagnostics and use laziness and linkage
to determine if a function is emitted (for the device). Note that we
still eagerly emit diagnostics for target regions, unfortunately, see
the TODO for the reason.
This hopefully fixes PR48933.
Reviewed By: JonChesterfield
Differential Revision: https://reviews.llvm.org/D95928