[OPENMP50]Support 'task' modifier in reduction clauses.

Summary:
Added basic support for 'task' modifier in the reduction clauses in
non-simd parallel and worksharing constructs.

Reviewers: jdoerfert

Subscribers: yaxunl, guansong, cfe-commits, caomhin

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78738