I think OffloadAction::DeviceDependences::add(..., ..., /*BoundArch=*/nullptr, Action::OFK_OpenMP) is never sufficient. The invalid BoundArch eventually ends up in NVPTX::Assembler::ConstructJob and triggers an assert; I don't think there is any code path with OpenMP offloading where the GPU architecture is set correctly. If I compile a simple test file with

clang -fopenmp=libomp -fopenmp-targets=nvptx64-nvidia-cuda -c example.c -march=sm_30

the error message is the following:

clang: /llvm/tools/clang/lib/Driver/Tools.cpp:11960: virtual void clang::driver::tools::NVPTX::Assembler::ConstructJob(clang::driver::Compilation&, const clang::driver::JobAction&, const clang::driver::InputInfo&, const InputInfoList&, const llvm::opt::ArgList&, const char*) const: Assertion `gpu_arch != CudaArch::UNKNOWN && "Device action expected to have an architecture."' failed.

The three smaller patches into which you divided this one appear to be missing some things. For example, AddOpenMPLinkerScript in lib/Driver/Tools.cpp from this patch appears to still be necessary to get the desired functionality, but it is not present in any of the three.