Try run() instead of command_output()

Author: Oneirical

src/tools/run-make-support/src/llvm.rs

@@ -118,7 +118,7 @@ impl LlvmFilecheck {
     /// Construct a new `llvm-filecheck` invocation. This assumes that `llvm-filecheck` is available
     /// at `$LLVM_FILECHECK`.
     pub fn new() -> Self {
-        let llvm_filecheck = env_var("LLVM_FILECHECK").expect("LLVM_FILECHECK env var not specified");
+        let llvm_filecheck = env_var("LLVM_FILECHECK");
         let cmd = Command::new(llvm_filecheck);
         Self { cmd }
     }