[openmp][libomptarget] Include header from LLVM source tree

The change is to the amdgpu plugin so is unlikely to break anything.

The point of contention is whether libomptarget can depend on LLVM.
A community discussion was cautiously not opposed yesterday.

This introduces a compile time dependency on the LLVM source tree, in this case
expressed as skipping the building of the plugin if LLVM_MAIN_INCLUDE_DIR is not
set. One the source files will #include llvm/Frontend/OpenMP/OMPGridValues.h,
instead of copy&pasting the numbers across.

For users that download the monorepo, the llvm tree is already on disk. This will
inconvenience users who download only the openmp source as a tar, as they would
now also have to download (at least a file or two) from the llvm source, if they want
to build the parts of the openmp project that (post this patch) depend on llvm.

There was interest expressed in going further - using llvm tools as part of
building libomp, or linking against llvm libraries. That seems less clear cut
an improvement and worthy of further discussion. This patch seeks only to change
policy to support openmp depending on the llvm source tree. Including in the
other direction, or using libraries / tools etc, are purposefully out of scope.

Reviewers are a best guess at interested parties, please feel free to add others

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D87841