libstdc++/4150: catastrophic performance decrease in C++ code

[Benjamin Kosnik]

> Done.

Great. Thanks again for all your efforts.

Now, I believe this fixes the current 'high' priority GNATS bugs, both
4150 and 5396. If so, can you put them into feedback or close them? 
It would be appreciated.

Loren, I think you had a patch on top of Jason's. Can you give status
please?

I'll start working on the intractivity bug, represented by 5820.

-benjamin