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

The three switch fallthrough generate a warning with -Wimplicit-fallthrough.
Two are documented as fallthrough, one is not, but I think the intention is to also fallthrough in kmp_tasking.cpp.

Not sure whether kmp.h is the best place to define the macro.

Reviewers: jlpeyton, AndreyChurbanov, Hahnfeld

Reviewed By: jlpeyton

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D56397

llvm-svn: 353052