This adds a workspace rule that inspects the current system first and
downloads a suitable version of LLVM/Clang from GitHub if it can't
find one. In the latter case, the necessary parts are build from source,
which can take a while (~10-15m, depending on the build machine).
In order to be found, LLVM/Clang system libraries must be version 11
or higher. On Debian/Ubuntu, install `llvm-13-dev` and `libclang-13-dev`.
The new `llvm_config.bzl` implements this logic. It is loosely based on
upstream's https://github.com/llvm/llvm-project/blob/main/utils/bazel/configure.bzl.
Note that due to the way Bazel separates local repositories, we have to
duplictate some of this code.
PiperOrigin-RevId: 438759950
Change-Id: Ia65f473b4cdef6507e3816bf09794ea10963d87a
