[CUDA] Improve clang's ability to detect recent CUDA versions.

CUDA-11.1 does not carry version.txt which causes clang to assume that it's
CUDA-7.0, which used to be the only CUDA version w/o version.txt.

In order to tell CUDA-7.0 apart from the new versions, clang now probes for the
presence of libdevice.10.bc which is not present in the old CUDA versions.

This should keep Clang working for CUDA-11.1.

PR47332: https://bugs.llvm.org/show_bug.cgi?id=47332

Differential Revision: https://reviews.llvm.org/D89752