Make combining LLD with external LLVM config a hard error

Author: Kobzol

src/bootstrap/src/core/build_steps/compile.rs

@@ -2241,7 +2241,7 @@ 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 && !builder.config.is_system_llvm(target_compiler.host) {
+        if builder.config.lld_enabled {
             builder.ensure(crate::core::build_steps::tool::LldWrapper {
                 build_compiler,
                 target_compiler,

src/bootstrap/src/core/config/config.rs

@@ -1003,9 +1003,7 @@ impl Config {
         }
 
         if config.lld_enabled && config.is_system_llvm(config.host_target) {
-            eprintln!(
-                "Warning: LLD is enabled when using external llvm-config. LLD will not be built and copied to the sysroot."
-            );
+            panic!("Cannot enable LLD with `rust.lld = true` when using external llvm-config.");
         }
 
         config.optimized_compiler_builtins =