[OpenMP] Modernize the kernel launching interface and APIs

We already created a versioned `__tgt_kernel_arguments` struct but it
was only briefly used and its content was passed in isolation anyway.
This makes it hard to add more information in the future. With this
patch we fully embrace the struct as means to pass information from the
compiler to the plugin as part of a kernel launch.

The patch also extends and renames the struct, bumping the version
number to 2. Version 1 entries are auto-upgraded. This is in preparation
for "bare" kernel launches, per kernel dynamic shared memory, CUDA/HIP
lowering, etc.

The `__tgt_target_kernel_nowait` interface was deprecated as it was
unused. Once we actually implement support for something like that, we
can add an appropriate API.

Note: Only plugins with the `launch_kernel` interface are now supported.
      That means that a new clang won't be able to use an old runtime.
      An old clang can still use the new runtime since the libomptarget
      interface did not change.

Differential Revision: https://reviews.llvm.org/D141232