Add support for bazel-contrib/toolchains_llvm

[scasagrande]

When using bazel-contrib/toolchains_llvm and specifying:

toolchains = {
    "cc_toolchain": "@bazel_tools//tools/cpp:current_cc_toolchain",
},

You end up with the following error:

Error in fail: toolchain target @bazel_tools//tools/cpp:current_cc_toolchain for 'cc_toolchain' has files from different repositories: external/toolchains_llvm~~llvm~llvm_toolchain_llvm, external/toolchains_llvm~~llvm~llvm_toolchain

However the second one actually just points back to the first:

❯ ls -la /private/var/tmp/_bazel_steven/d71d77490eeb3a6c5a6efc45dc1ed897/external/toolchains_llvm\~\~llvm\~llvm_toolchain/bin
total 16
drwxr-xr-x@ 14 steven  wheel   448 Aug 31 13:21 .
drwxr-xr-x@  7 steven  wheel   224 Aug 31 13:21 ..
-rwxr-xr-x@  1 steven  wheel  6516 Aug 31 13:21 cc_wrapper.sh
lrwxr-xr-x@  1 steven  wheel   128 Aug 31 13:21 clang-cpp -> /private/var/tmp/_bazel_steven/d71d77490eeb3a6c5a6efc45dc1ed897/external/toolchains_llvm~~llvm~llvm_toolchain_llvm/bin/clang-cpp
lrwxr-xr-x@  1 steven  wheel   125 Aug 31 13:21 ld.lld -> /private/var/tmp/_bazel_steven/d71d77490eeb3a6c5a6efc45dc1ed897/external/toolchains_llvm~~llvm~llvm_toolchain_llvm/bin/ld.lld
lrwxr-xr-x@  1 steven  wheel   138 Aug 31 13:21 libtool -> /private/var/tmp/_bazel_steven/d71d77490eeb3a6c5a6efc45dc1ed897/external/toolchains_llvm~~llvm~llvm_toolchain_llvm/bin/llvm-libtool-darwin
<trimmed>

My current implementation here is just to add a special case for toolchains_llvm, however I understand that's probably not ideal. I'm open to better ideas.

Then there is the question of testing. I can add toolchains_llvm to examples/ but we'll have to download a large pre-built llvm package as part of CI. Let me know what you'd like to see.

[fmeum]

Thanks for the detailed description of the problem!

I would like to take some time to think of a cleaner approach than hardcoding repos.

[scasagrande]

Yeah, I don't like it either. Take your time, let me know if you come up with something.