Fix x test src/tools/error_index_generator --stage {0,1}

There were two fixes needed:
1. Use `top_stage` instead of `top_stage - 1`. There was a long and torturous comment about trying to match rustdoc's version, but it works better without the hard-coding than with.
2. Make sure that `ci-llvm/lib` is added to LD_LIBRARY_PATH. Previously the error index would be unable to load LLVM for stage0 builds.

At some point we should probably have a discussion about how rustdoc stages should be numbered;
confusion between 0/1/2 has come up several times in bootstrap now.

Note that this is still broken when using `download-rustc = true` and `--stage 1`,
but that's *really* a corner case and should affect almost no one. `--stage {0,2}`
work fine with download-rustc.

src/bootstrap/builder.rs

@@ -835,6 +835,18 @@ impl<'a> Builder<'a> {
         }
     }
 
+    pub fn rustc_lib_paths(&self, compiler: Compiler) -> Vec<PathBuf> {
+        let mut dylib_dirs = vec![self.rustc_libdir(compiler)];
+
+        // Ensure that the downloaded LLVM libraries can be found.
+        if self.config.llvm_from_ci {
+            let ci_llvm_lib = self.out.join(&*compiler.host.triple).join("ci-llvm").join("lib");
+            dylib_dirs.push(ci_llvm_lib);
+        }
+
+        dylib_dirs
+    }
+
     /// Adds the compiler's directory of dynamic libraries to `cmd`'s dynamic
     /// library lookup path.
     pub fn add_rustc_lib_path(&self, compiler: Compiler, cmd: &mut Command) {
@@ -845,15 +857,7 @@ impl<'a> Builder<'a> {
             return;
         }
 
-        let mut dylib_dirs = vec![self.rustc_libdir(compiler)];
-
-        // Ensure that the downloaded LLVM libraries can be found.
-        if self.config.llvm_from_ci {
-            let ci_llvm_lib = self.out.join(&*compiler.host.triple).join("ci-llvm").join("lib");
-            dylib_dirs.push(ci_llvm_lib);
-        }
-
-        add_dylib_path(dylib_dirs, cmd);
+        add_dylib_path(self.rustc_lib_paths(compiler), cmd);
     }
 
     /// Gets a path to the compiler specified.

src/bootstrap/tool.rs

@@ -379,22 +379,14 @@ pub struct ErrorIndex {
 
 impl ErrorIndex {
     pub fn command(builder: &Builder<'_>) -> Command {
-        // This uses stage-1 to match the behavior of building rustdoc.
-        // Error-index-generator links with the rustdoc library, so we want to
-        // use the same librustdoc to avoid building rustdoc twice (and to
-        // avoid building the compiler an extra time). This uses
-        // saturating_sub to deal with building with stage 0. (Using stage 0
-        // isn't recommended, since it will fail if any new error index tests
-        // use new syntax, but it should work otherwise.)
-        let compiler = builder.compiler(builder.top_stage.saturating_sub(1), builder.config.build);
+        // Error-index-generator links with the rustdoc library, so we need to add `rustc_lib_paths`
+        // for rustc_private and libLLVM.so, and `sysroot_lib` for libstd, etc.
+        let host = builder.config.build;
+        let compiler = builder.compiler_for(builder.top_stage, host, host);
         let mut cmd = Command::new(builder.ensure(ErrorIndex { compiler }));
-        add_dylib_path(
-            vec![
-                PathBuf::from(&builder.sysroot_libdir(compiler, compiler.host)),
-                builder.rustc_libdir(compiler),
-            ],
-            &mut cmd,
-        );
+        let mut dylib_paths = builder.rustc_lib_paths(compiler);
+        dylib_paths.push(PathBuf::from(&builder.sysroot_libdir(compiler, compiler.host)));
+        add_dylib_path(dylib_paths, &mut cmd);
         cmd
     }
 }

src/bootstrap/util.rs

@@ -63,7 +63,7 @@ pub fn libdir(target: TargetSelection) -> &'static str {
 }
 
 /// Adds a list of lookup paths to `cmd`'s dynamic library lookup path.
-/// If The dylib_path_par is already set for this cmd, the old value will be overwritten!
+/// If the dylib_path_var is already set for this cmd, the old value will be overwritten!
 pub fn add_dylib_path(path: Vec<PathBuf>, cmd: &mut Command) {
     let mut list = dylib_path();
     for path in path {