[OpenCL] Allow function declaration with empty argument list.

Treat 'f()' as 'f(void)' rather than a function w/o a prototype.

Reviewers: Anastasia, yaxunl

Reviewed By: Anastasia, yaxunl

Subscribers: cfe-commits, echuraev, chapuni

Differential Revision: https://reviews.llvm.org/D33681

Re-apply revision 306653.

llvm-svn: 315453