[OpenMP] Add support for registering requires directives with the runtime

Summary:
This patch adds support for the registration of the requires directives with the runtime.

Each requires directive clause will enable a particular flag to be set.

The set of flags is passed to the runtime to be checked for compatibility with other such flags coming from other object files.

The registration function is called whenever OpenMP is present even if a requires directive is not present. This helps detect cases in which requires directives are used inconsistently.

Reviewers: ABataev, AlexEichenberger, caomhin

Reviewed By: ABataev, AlexEichenberger

Subscribers: jholewinski, guansong, jfb, jdoerfert, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D60568

llvm-svn: 361298