add coverage for llvm-config path resolution

Signed-off-by: onur-ozkan <work@onurozkan.dev>

src/bootstrap/src/core/build_steps/llvm.rs

@@ -54,6 +54,14 @@ impl LlvmBuildStatus {
             LlvmBuildStatus::ShouldBuild(_) => true,
         }
     }
+
+    #[cfg(test)]
+    pub fn llvm_result(&self) -> &LlvmResult {
+        match self {
+            LlvmBuildStatus::AlreadyBuilt(res) => res,
+            LlvmBuildStatus::ShouldBuild(meta) => &meta.res,
+        }
+    }
 }
 
 /// Linker flags to pass to LLVM's CMake invocation.
@@ -120,12 +128,17 @@ pub fn prebuilt_llvm_config(
     let root = "src/llvm-project/llvm";
     let out_dir = builder.llvm_out(target);
 
-    let build_llvm_config = if target == builder.config.build {
+    let build_llvm_config = if let Some(build_llvm_config) = builder
+        .config
+        .target_config
+        .get(&builder.config.build)
+        .and_then(|config| config.llvm_config.clone())
+    {
+        build_llvm_config
+    } else {
         let mut llvm_config_ret_dir = builder.llvm_out(builder.config.build);
         llvm_config_ret_dir.push("bin");
         llvm_config_ret_dir.join(exe("llvm-config", builder.config.build))
-    } else {
-        builder.ensure(Llvm { target: builder.config.build }).llvm_config
     };
 
     let llvm_cmake_dir = out_dir.join("lib/cmake/llvm");