[OpenMP][NFC] Generalize CGOpenMPRuntimeNVPTX as CGOpenMPRuntimeGPU

Refactors CGOpenMPRuntimeNVPTX as CGOpenMPRuntimeGPU to make it a
generalization for OpenMP GPU Codegen. Target specific specialized
methods for NVPTX are defined in class CGOpenMPRuntimeNVPTX. This
paves the way for a clean and maintainable extension to more GPU
targets for OpenMP Codegen.

For original author (git blame) list of CGOpenMPRuntimeGPU code,
look in history of CGOpenMPRuntimeNVPTX.cpp and .h, after this commit.

Reviewed By: ABataev

Differential Revision: https://reviews.llvm.org/D83723