placeholders shouldn't be marked with an unknown module

Placeholders shouldn't be marked with an unknown module.

I think we should have one or more synthetic modules for placeholders that has placeholders directly as children rather than having placeholders appearing as shown below: <unknown module> <unknown file> <placeholder 1> <placeholder 2>

A second issue is that in the flat view, placeholders such as <omp tgt copyin> show up as siblings of placeholders <gpu copyin>, which makes it look like the GPU copy cost is twice what it should be.

This would be fixed if there were separate placeholder modules for omp and gpu.