This PR enables `func::ConstantOp` creation and usage for device
functions inside GPU modules.
The current main returns error for referencing device functions via
`func::ConstantOp`, because during the `ConstantOp` verification it only
checks symbols in `ModuleOp` symbol table, which, of course, does not
contain device functions that are defined in `GPUModuleOp`. This PR
proposes a more general solution.
Co-authored-by: Artem Kroviakov <artem.kroviakov@tum.de>
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