[OpenMP][OMPT] Distinguish different barrier kinds

This change makes the runtime decide the intended use of each barrier
invocation, for the OMPT synchronization tool callbacks.  The OpenMP 5.0
specification defines four possible barrier kinds -- implicit, explicit,
implementation, and just normal barrier.

Patch by Hansang Bae

Differential Revision: https://reviews.llvm.org/D58247

llvm-svn: 355140