LLVM_SYMBOLIZER_PATH is required if `llvm-symbolizer` isn't in the developer's PATH. This sets it via bazel instead of having a developer handle it. I noticed this because the apt install of clang doesn't put llvm-symbolizer in the PATH.
I'd like to make this the default without putting it everywhere, but I don't see a way to do this intrinsically through [the toolchain](https://bazel.build/docs/cc-toolchain-config-reference), and the [rules_cc/defs.bzl](https://github.com/bazelbuild/rules_cc/blob/main/cc/defs.bzl) remains a thin wrapper around the native cc_binary.
Since I'm adding another env, it seems undesirable to have the macos asan workaround separate. As a consequence, this merges it in. Note bazel doesn't support merging a dict and a select, so it's also necessary to have the two env vars at least mildly aware there's something up (and this could get worse if we end up having more selects).
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.
Chandler Carruth
Jon Ross-Perkins