This commit builds on and completes the work done in
9f6c632ecd to eliminate the need for a
separate mlir-spirv-cpu-runner binary. Since the MLIR processing is
already done outside this runner, the only real difference between it
and the mlir-cpu-runner is the final linking step between the nested
LLVM IR modules. By moving this step into mlir-cpu-runner behind a new
command-line flag (`--link-nested-modules`), this commit is able to
completely remove the runner component of the mlir-spirv-cpu-runner.
The runtime libraries and the tests are moved and renamed to fit into
the Execution Engine and Integration tests, following the model of the
similar migration done for the CUDA Runner in D97463.
0e39b1348e