[NFC][OpenMP] Prepare amdgpu plugin for asynchronous implementation of target region launch

At present, amdgpu plugin merges both asynchronous and synchronous kernel launch implementations into a single synchronous version.
This patch prepares the plugin for asynchronous implementation by:
- Privatizing actual kernel launch code (valid in both cases) into an anonymous namespace base function

Actual separation of kernel launch code (async vs sync) is a following patch.

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D115267