[OpenMP] Part 2 of At present, amdgpu plugin merges both asynchronous

[OpenMP] Part 2 of 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 (submitted at D115267)

    - Separating the control flow path of asynchronous and synchronous
      kernel launch functions** (this diff)

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D115273