Rollup merge of #131470 - EnzymeAD:enzyme-testinfra2, r=jieyouxu

add test infra to explicitely test rustc with autodiff/enzyme disabled

I assume this is not what you want for now, but I'll update the PR once I understand how the ignore- directives work.

To summarize the situation, we want a feature gate test where we don't enable the autodiff feature using `#![feature(autodiff)]`. There are two situations.
1) We have a rustc which was build without autodiff support (current default): It gives one error about the feature being needed and one error about this rustc version being build without autodiff support.
2) We have a rustc which was build with autodiff support (i.e. for now a custom build): It gives one error about the feature being needed.

We have a `//```@needs-enzyme```` directive which we can use in revisions for the second case.
However, we have no way to specify that needs-enzyme implies that the second error should not be seen.
This ads a way of passing the following test:
```
//@ revisions: has_support no_support
//```@[has_support]``` needs-enzyme
//```@[no_support]``` needs-enzyme-disabled

#![crate_type = "lib"]

#[autodiff(dfoo, Reverse)]
//[has_support]~^ ERROR use of unstable library feature 'autodiff' [E0658]
//[no_support]~^^ ERROR use of unstable library feature 'autodiff' [E0658]
//[no_support]~| ERROR this rustc version does not support autodiff
fn foo() {}
```
Cherry picking this PR to my frontend pr makes the test above pass in both configurations (enzyme=true/false in config.toml).
I'm open to other changes that make this testcase pass.

r? ```@jieyouxu```

Tracking:

- #124509

src/tools/compiletest/src/command-list.rs

@@ -49,6 +49,7 @@ const KNOWN_DIRECTIVE_NAMES: &[&str] = &[
     "ignore-eabi",
     "ignore-emscripten",
     "ignore-endian-big",
+    "ignore-enzyme",
     "ignore-freebsd",
     "ignore-fuchsia",
     "ignore-gdb",

src/tools/compiletest/src/header/cfg.rs

@@ -166,6 +166,12 @@ pub(super) fn parse_cfg_name_directive<'a>(
         message: "when the target vendor is Apple"
     }
 
+    condition! {
+        name: "enzyme",
+        condition: config.has_enzyme,
+        message: "when rustc is built with LLVM Enzyme"
+    }
+
     // Technically the locally built compiler uses the "dev" channel rather than the "nightly"
     // channel, even though most people don't know or won't care about it. To avoid confusion, we
     // treat the "dev" channel as the "nightly" channel when processing the directive.