Check for llvm-tools before install

Author: chaitanyav

src/bootstrap/install.rs

@@ -210,10 +210,13 @@ install!((self, builder, _config),
         }
     };
     LlvmTools, alias = "llvm-tools", Self::should_build(_config), only_hosts: true, {
-        let tarball = builder
-            .ensure(dist::LlvmTools { target: self.target })
-            .expect("missing llvm-tools");
-        install_sh(builder, "llvm-tools", self.compiler.stage, Some(self.target), &tarball);
+        if let Some(tarball) = builder.ensure(dist::LlvmTools { target: self.target }) {
+            install_sh(builder, "llvm-tools", self.compiler.stage, Some(self.target), &tarball);
+        } else {
+            builder.info(
+                &format!("skipping llvm-tools stage{} ({}): external LLVM", self.compiler.stage, self.target),
+            );
+        }
     };
     Rustfmt, alias = "rustfmt", Self::should_build(_config), only_hosts: true, {
         if let Some(tarball) = builder.ensure(dist::Rustfmt {