[OpenCL] Reject reqd_work_group_size(X, Y, Z) where X, Y or Z == 0.

Patch by Pedro Ferreira!


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209127 91177308-0d34-0410-b5e6-96231b3b80d8
Fixing some comments in this test. No functional changes.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209128 91177308-0d34-0410-b5e6-96231b3b80d8