If CUDA version can not be determined based on version.txt file, attempt to find
CUDA_VERSION macro in cuda.h.
This is a follow-up to D89752,
Differntial Revision: https://reviews.llvm.org/D89832
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
Artem Belevich