[CUDA] Allow local static variables with target attributes.

While CUDA documentation claims that such variables are not allowed[1], NVCC has
been accepting them since CUDA-10.0[2] and some headers in CUDA-11 rely on this
working.

1. https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#static-variables-function
2. https://godbolt.org/z/zsodzc

Differential Revision: https://reviews.llvm.org/D88345