Disable combining LLD with external llvm-config

Author: Kobzol

Diff for: src/bootstrap/src/core/build_steps/compile.rs

@@ -2182,7 +2182,7 @@ impl Step for Assemble {
         debug!("copying codegen backends to sysroot");
         copy_codegen_backends_to_sysroot(builder, build_compiler, target_compiler);
 
-        if builder.config.lld_enabled {
+        if builder.config.lld_enabled && !builder.is_system_llvm(target_compiler.host) {
             builder.ensure(crate::core::build_steps::tool::LldWrapper {
                 build_compiler,
                 target_compiler,

Diff for: src/bootstrap/src/core/config/config.rs

@@ -2397,6 +2397,15 @@ impl Config {
             );
         }
 
+        if config
+            .target_config
+            .get(&config.build)
+            .is_some_and(|config| config.llvm_config.is_some())
+            && config.lld_enabled
+        {
+            panic!("Cannot enable LLD when using external llvm-config.");
+        }
+
         let default_std_features = BTreeSet::from([String::from("panic-unwind")]);
         config.rust_std_features = std_features.unwrap_or(default_std_features);