This updates the Bazel toolchain logic to work on macOS. Much like on
Linux, I'm not testing this against a *released* LLVM, but against
a from-source build. You can build and install LLVM from top-of-tree
locally with CMake, or on macOS maybe use Homebrew like:
```
brew install llvm --HEAD
```
You then need to point Bazel to the installed `clang` executable if it
is not placed onto your PATH (Homebrew doesn't):
```
bazel test --repo_env=CC=$HOME/homebrew/opt/llvm/bin/clang //parser:all
```
This builds and passes tests for me at least.
The fuzzer feature may not work (yet) with this setup, but that can be
improved incrementally as we proceed.
The Bazel bits are collected into a directory and given less confusing
names (I hope). Other than names, everything is a direct copy from the
toolchain repository without any edits.
A few files needed to be merged in:
- `.gitignore`
- `.pre-commit-config.yaml`
Subsequent commits will add relevant C++ infrastructure and then the
source code itself.
Jon Meow
