Auto merge of #110478 - jyn514:stage1-fulldeps, r=albertlarsan68

Support `x test --stage 1 ui-fulldeps`

`@Nilstrieb` had an excellent idea the other day: the same way that rustdoc is able to load `rustc_driver` from the sysroot, ui-fulldeps tests should also be able to load it from the sysroot. That allows us to run fulldeps tests with stage1, without having to fully rebuild the compiler twice. It does unfortunately have the downside that we're building the tests with the *bootstrap* compiler, not the in-tree sources, but since most of the fulldeps tests are for the *API* of the compiler, that seems ok.

I think it's possible to extend this to `run-make-fulldeps`, but I've run out of energy for tonight.

- Move `plugin` tests into a subdirectory.

  Plugins are loaded at runtime with `dlopen` and so require the ABI of the running compile to match the ABI of the compiler linked with `rustc_driver`. As a result they can't be supported in stage 1 and have to use `// ignore-stage1`.

- Remove `ignore-stage1` from most non-plugin tests

- Ignore diagnostic tests in stage 1. Even though this requires a stage 2 build to load rustc_driver, it's primarily testing the error message that the *running* compiler emits when the diagnostic struct is malformed.

- Pass `-Zforce-unstable-if-unmarked` in stage1, not just stage2. That allows running `hash-stable-is-unstable` in stage1, since it now suggests adding `rustc_private` to enable loading the crates.

- Add libLLVM.so to the stage0 target sysroot, to allow fulldeps tests that act as custom drivers to load it at runtime.

- Pass `--sysroot stage0-sysroot` in compiletest so that we use the correct version of std.

- Move a few lint tests from ui-fulldeps to ui

  These had an `aux-build:lint-group-plugin-test.rs` that they never actually loaded with `feature(plugin)` nor tested. I removed the unused aux-build and they pass fine with stage 1.

Fixes rust-lang/rust#75905.