[CUDA] Disallow 'extern __shared__' variables.

Also add a test that we disallow

  __constant__ __shared__ int x;

because it's possible to break this without breaking

  __shared__ __constant__ int x;

Reviewers: rnk

Subscribers: cfe-commits, tra

Differential Revision: https://reviews.llvm.org/D25125

llvm-svn: 282985