[OpenMP] Use C++11 Atomics - barrier, tasking, and lock code

These are preliminary changes that attempt to use C++11 Atomics in the runtime.
We are expecting better portability with this change across architectures/OSes.
Here is the summary of the changes.

Most variables that need synchronization operation were converted to generic
atomic variables (std::atomic<T>). Variables that are updated with combined CAS
are packed into a single atomic variable, and partial read/write is done
through unpacking/packing

Patch by Hansang Bae

Differential Revision: https://reviews.llvm.org/D47903

llvm-svn: 336563