[OpenMP] Upgrade default version of OpenMP to 5.0

Summary:
When -fopenmp option is specified then version 5.0 will be set as
default.

Reviewers: gregrodgers, jdoerfert, ABataev

Reviewed By: ABataev

Subscribers: pdhaliwal, yaxunl, guansong, sstefan1, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D81098