[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.
git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@315453 91177308-0d34-0410-b5e6-96231b3b80d8