[flang] CUDA Fortran - part 3/5: declarations checking

Implements checks for CUDA Fortran attributes on objects, types, and
subprograms.  Includes a couple downgrades of existing errors into
warnings that were exposed during testing.

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

Differential Revision: https://reviews.llvm.org/D150162