Architecture 30 was removed with CUDA 11, so most of the CUDA tests fail with
it.
Remove setting the architecture and bump the minimum version to 3.18, so
CMP0104 takes effect and we can rely on the default architecture, which is
guaranteed to be compilable.
Use of __ldg() in ProperLinkFlags was removed as it only affects performance
and is available only on sm_35 and above.
Testing the functionality of CUDA_ARCHITECTURES is already covered by
CudaOnly.Architecture and CudaOnly.CompileFlags.
In commit a653ca9504 (Tests: Update CUDA tests to work with Clang,
2020-03-27) some tests were conditioned using `CMAKE_CUDA_COMPILER_ID`.
That is not defined when configuring CMake itself, so it accidentally
turned off NVCC-specific CUDA tests altogether. Convert the conditions
to check `CMake_TEST_CUDA` for `Clang` instead. That option is added
explicitly to builds where we want the tests to run, so we can set it to
a value indicating the CUDA compiler vendor.
In commit a653ca9504 (Tests: Update CUDA tests to work with Clang,
2020-03-27) the NVCC-specific `CudaOnly.GPUDebugFlag` test was
accidentally broken by removing a space when appending `-G` to the CUDA
flags. This was covered by the test not running. Restore the space.
Fixes: #20727
