[OMPT] Make omp_control_tool() compliant when called from Fortran programs

This change fixes an incorrect behavior of the omp_control_tool function when
called from Fortran applications.  A tool callback function for this event is
supposed to get NULL for the third argument according to the specification, but
the current implementation just passes a garbage value. A possible fix is to use
the OPTIONAL attribute for the third argument.

Patch by Hansang Bae

Differential Revision: https://reviews.llvm.org/D50565

llvm-svn: 339585