string debug-mode solutions

[Benjamin Kosnik]

> If the symbols are weak there's no problem with ODR, right?

Right.

So, I'll do this unless I hear loud screams soon.

When you do the doc updates, can you also add something about the
multiple translation units linked together thing? In this way, debug
string is different than other containers, which have __gnu_norm and
__gnu_debug namespace mangling to protect them. 

-benjamin