[flang] CUDA Fortran - part 5/5: statement semantics

Canonicalize !$CUF KERNEL DO loop nests, similar to OpenACC/OpenMP
canonicalization.  Check statements and expressions in device contexts
for usage that isn't supported.  Add more tests, and include some
tweaks to standard modules needed to build CUDA Fortran modules.

Depends on https://reviews.llvm.org/D150159,
https://reviews.llvm.org/D150161, https://reviews.llvm.org/D150162, &
https://reviews.llvm.org/D150163.

Differential Revision: https://reviews.llvm.org/D150164