[OpenMP][Fix] Conditional compilation leaves variables unused

Summary: Prevent variables from being left unused by conditional compilation.

Reviewers: ABataev, grokos, Hahnfeld, caomhin, protze.joachim

Reviewed By: Hahnfeld

Subscribers: guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D51303

llvm-svn: 340771