[llvm] Fix bug leading to zombie llvm-size processes.

This corrects my use of the boost::process API whereby llvm-size child
processes could become detached from the parent environment service
and be left as zombies.

I found this issue will attempting to reproduce facebookresearch#572. I'm not sure if
this bug is relevant.

compiler_gym/envs/llvm/service/Cost.cc

@@ -113,20 +113,21 @@ Status getTextSizeInBytes(llvm::Module& module, int64_t* value, const fs::path&
  bp::child llvmSize(llvmSizeCmd, bp::std_in.close(), bp::std_out > llvmSizeStdoutFuture,
  bp::std_err > bp::null, llvmSizeStdoutStream);
 
- if (!util::wait_for(llvmSize, std::chrono::seconds(60))) {
+ llvmSizeStdoutStream.run_for(std::chrono::seconds(60));
+ if (llvmSizeStdoutStream.poll()) {
  return Status(StatusCode::DEADLINE_EXCEEDED,
  fmt::format("Failed to compute .text size cost within 60 seconds"));
  }
+ llvmSize.wait();
+ llvmSizeOutput = llvmSizeStdoutFuture.get();
 
- llvmSizeStdoutStream.run();
  fs::remove(tmpFile);
  if (llvmSize.exit_code()) {
  return Status(StatusCode::INVALID_ARGUMENT, fmt::format("Failed to compute .text size cost. "
  "Command returned exit code {}: {}",
  llvmSize.exit_code(), llvmSizeCmd));
  }
 
- llvmSizeOutput = llvmSizeStdoutFuture.get();
  } catch (bp::process_error& e) {
  fs::remove(tmpFile);
  return Status(StatusCode::INVALID_ARGUMENT,
@@ -154,6 +155,7 @@ Status getTextSizeInBytes(llvm::Module& module, int64_t* value, const fs::path&
  return Status(StatusCode::INTERNAL,
  fmt::format("Failed to parse .TEXT size: `{}`\n", llvmSizeOutput));
  }
+
  return Status::OK;
 }