In sun or shade? Exposed to cold winds, or sheltered? How big is the bunker?

And how high is the wall above the bunker?

Two choices depending on the situation would be Ceanothus or Escallonia.

I take it you're after a climber rather than a shrub...? If so, and it must be evergreen and self-supporting, then you really only have two choices: An ivy, or Hydrangea Seemanii. This hydrangea is slow-growing, but romps away once it gets going. It would be my choice as it has gorgeous white flowers. I have one on a north-facing wall, and I grow clematis through it for extended interest.

If you'd rather have a shrub, then probably best to avoid Escallonia. They're being decimated nationwide by Escallonia Leaf Spot, which is currently untreatable. We can't really advise you on what other shrubs to grow until we know the aspect and soil type.