[OpenMP] Use `add_llvm_library` to build the target `PluginInterface` in `plugins-nextgen`

This patch uses `add_llvm_library` to build the target `PluginInterface` since it can handle LLVM dependences much better. One temporary drawback of using this is that currently LLVM CMake macro doesn't support object libraries very well (there was a try a couple years ago but it was reverted later https://github.com/llvm/llvm-project/commit/29e57229497711a3a294f437b59afa6ddc36a3d8). After switching to that, `CXX_VISIBILITY_PRESET` can not be set correctly, which can cause runtime error that a function call from one plugin could go to another. As a consequence, `PluginInterface` is built as a static library for now. I have asked the question in CMake community (https://discourse.cmake.org/t/set-target-properties-doesnt-work-properly/7016). Once that issue is solved, I'll switch it back to object library. It is not necessarily too bad to use static library, especially `BUILDTREE_ONLY` is already set such that `PluginInterface.a` will not be installed.

Reviewed By: jhuber6

Differential Revision: https://reviews.llvm.org/D139371