devel/oclgrind: Update LLVM dependency to LLVM5 to stay in lockstep with the other OpenCL ports (see PR). In my testing, this also requires a newer development snapshot for runtime stability - so update to latest snapshot.

PR: 224652
Submitted by: jbeich (for the LLVM update)
Reviewed by: swills (mentor)
Approved by: swills (mentor)
Differential Revision: https://reviews.freebsd.org/D13726