[OpenMP] Refined CUDA plugin to put all CUDA operations into class

Summary: Current implementation mixed everything up so that there is almost no encapsulation. In this patch, all CUDA related operations are put into a new class DeviceRTLTy and only necessary functions are exposed. In addition, all C++ code now conforms with LLVM code standard, keeping those API functions following C style.

Reviewers: jdoerfert

Reviewed By: jdoerfert

Subscribers: jfb, yaxunl, guansong, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D77951