Add support for Clang-CUDA 11.x and a post-merge CI build

Add support for Clang-CUDA 11.x and a post-merge CI build

Tested mostly with Clang-11 and CUDA-11.2

While Clang does not officially support CUDA after 10.1, it appears to
work fine with newer versions.

Caveat: Clang fails to properly detect CUDA versions past 10.2

This can be worked around by creating `/usr/local/cuda/version.txt`
file with the single line `CUDA Version X.Y.Z`.

Despite the workaround above, Clang:
- does not recognize sm_86.
- recognizes sm_80, but believes that the installed CUDA version
  is too old to support it.