Use LLVMDisposePassManager instead of raw delete in rustllvm

src/rustllvm/PassWrapper.cpp

@@ -537,7 +537,7 @@ LLVMRustWriteOutputFile(LLVMTargetMachineRef Target, LLVMPassManagerRef PMR,
   // Apparently `addPassesToEmitFile` adds a pointer to our on-the-stack output
   // stream (OS), so the only real safe place to delete this is here? Don't we
   // wish this was written in Rust?
-  delete PM;
+  LLVMDisposePassManager(PMR);
   return LLVMRustResult::Success;
 }