This PR adds `SKIP` to `add_lit_testsuites`. The purpose is to let people filter the currently exhaustive inclusion of subdirectories which might not all depend on the same `DEPENDS`. The immediate use case is MLIR where we have a collection of Python tests that currently trigger rebuilds of various tools (like `mlir-opt`) which they do not depend on. That collection of tests is updated here too.
See docs/CMake.html for instructions on how to build LLVM with CMake.