[OMPT] Make sure that OMPT is enabled when accessing internals of the runtime

Make sure that OMPT is enabled in runtime entry points that access internals
of the runtime. Else, return an appropiate value indicating an error or that
the data is not available.

Patch provided by @sconvent

Reviewers: jlpeyton, omalyshe, hbae, Hahnfeld, joachim.protze

Reviewed By: joachim.protze

Tags: #openmp, #ompt

Differential Revision: https://reviews.llvm.org/D47717

llvm-svn: 351311