Rollup merge of #121347 - davidtwco:compiletest-aux-aux, r=oli-obk

compiletest: support auxiliaries with auxiliaries

To test behaviour that depends on the extern options of intermediate crates, compiletest auxiliaries must have their own auxiliaries.

Auxiliary compilation previously did not trigger compilation of any auxiliaries in the auxiliary's headers. In addition, those auxiliaries would need to be in an `auxiliary/auxiliary` directory, which is unnecessary and makes some crate graphs harder to write tests for, such as when A depends on B and C, and B depends on C.

For a test `tests/ui/$path/root.rs`, with the following crate graph:

```
root
|-- grandparent
`-- parent
    `-- grandparent
```

then the intermediate outputs from compiletest will be:

```
build/$target/test/ui/$path/
|-- auxiliary
|   |-- libgrandparent.dylib
|   |-- libparent.dylib
|   |-- grandparent
|   |   |-- grandparent.err
|   |   `-- grandparent.out
|   `-- parent
|       |-- parent.err
|       `-- parent.out
|-- libroot.rmeta
|-- root.err
`-- root.out
```

0 files changed, 0 insertions, 0 deletions