Auto merge of #86267 - ZuseZ4:master, r=nagisa

Allow loading of llvm plugins on nightly

Based on a discussion in  #82734 / with `@wsmoses.`
Mainly moves [this](0149bc4e7e) behind a -Z flag, so it can only be used on nightly,
as requested by `@nagisa` in https://github.com/rust-lang/rust/issues/82734#issuecomment-835863940

This change allows loading of llvm plugins like Enzyme.
Right now it also requires a shared library LLVM build of rustc for symbol resolution.

```rust
// test.rs
extern { fn __enzyme_autodiff(_: usize, ...) -> f64; }

fn square(x : f64) -> f64 {
   return x * x;
}

fn main() {
   unsafe {
      println!("Hello, world {} {}!", square(3.0), __enzyme_autodiff(square as usize, 3.0));
   }
}
```
```
./rustc test.rs -Z llvm-plugins="./LLVMEnzyme-12.so" -C passes="enzyme"
./test
Hello, world 9 6!
```

I will try to figure out how to simplify the usage and get this into stable in a later iteration,
but having this on nightly will already help testing further steps.

compiler/rustc_interface/src/tests.rs

@@ -715,6 +715,7 @@ fn test_debugging_options_tracking_hash() {
     tracked!(instrument_coverage, Some(InstrumentCoverage::All));
     tracked!(instrument_mcount, true);
     tracked!(link_only, true);
+    tracked!(llvm_plugins, vec![String::from("plugin_name")]);
     tracked!(merge_functions, Some(MergeFunctions::Disabled));
     tracked!(mir_emit_retag, true);
     tracked!(mir_opt_level, Some(4));