[OpenMP][CUDA] Cache the maximal number of threads per block (per kernel)

Instead of calling `cuFuncGetAttribute` with
`CU_FUNC_ATTRIBUTE_MAX_THREADS_PER_BLOCK` for every kernel invocation,
we can do it for the first one and cache the result as part of the
`KernelInfo` struct. The only functional change is that we now expect
`cuFuncGetAttribute` to succeed and otherwise propagate the error.
Ignoring any error seems like a slippery slope...

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D86038