When using toolchains_llvm the `parse_headers` feature appears to have no effect

[arvid-norlander]

According to https://bazel.build/docs/bazel-and-cpp#toolchain-features there are two supported "features" values:

• parse_headers
• layering_check

With toolchains_llvm it seems only the latter has any effect, parse_headers is silently ignored, doing nothing. It would be great if this could be supported.