[llvm] [bazel][libc] Add layering deps (PR #210380)

[Jordan Rupprecht via llvm-commits]

rupprecht wrote:

> Not sure why CI didn't catch it

It's actually a parse_headers issue, which isn't enabled by default:

```
libc/test/src/sys/socket/linux/socket_test_support.h:30:10: fatal error: 'test/UnitTest/LibcTest.h' file not found
   30 | #include "test/UnitTest/LibcTest.h"
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~
```

https://github.com/llvm/llvm-project/pull/210380