Auto merge of #9404 - ehuss:rustdoc-fingerprint-remove-dir, r=alexcrichton

Some changes to rustdoc fingerprint checking.

#8640 introduced a check which deletes the `doc` directory if cargo detects it has stale contents from a different toolchain version. Rustdoc has some shared files (js and css for example) that can get corrupted between versions. Unfortunately that caused some problems with rustbuild which does a few unusual things. Rustbuild will:

* Create the `doc` directory before running `cargo doc` and places a `.stamp` file inside it.
* Creates symlinks of the `doc` directory so that they can be shared across different target directories (in particular, between rustc and rustdoc).

In order to address these issues, this PR does several things:

* Adds `-Z skip-rustdoc-fingerprint` to disable the `doc` clearing behavior.
* Don't delete the `doc` directory if the rustdoc fingerprint is missing. This is intended to help with the scenario where the user creates a `doc` directory ahead of time with pre-existing contents before the first build. The downside is that cargo will not be able to protect against switching from pre-1.53 to post-1.53.
* Don't delete the `doc` directory itself (just its contents). This should help if the user created the `doc` directory as a symlink to somewhere else.
* Don't delete hidden files in the `doc` directory. This isn't something that rustdoc creates.

Only the `-Z` change is needed for rustbuild. The others I figured I'd include just to be on the safe side in case there are other users doing unusual things (and I had already written them thinking they would work for rustbuild). Hopefully the rustbuild `.stamp` mechanism will be enough protection there.

Fixes #9336