[CUDA] Allow "extern __shared__ Foo foo[]" within anon. namespaces.

Summary:
Previously this triggered a -Wundefined-internal warning.  But it's not
an undefined variable -- any variable of this form is a pointer to the
base of GPU core's shared memory.

Reviewers: tra

Subscribers: sanjoy, rsmith

Differential Revision: https://reviews.llvm.org/D46782

llvm-svn: 332621