[OPENMP50]Add initial support for 'affinity' clause.

Summary:
Added parsing/sema/serialization support for affinity clause in task
directives.

Reviewers: jdoerfert

Subscribers: yaxunl, guansong, arphaman, llvm-commits, cfe-commits, caomhin

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D80148