libstdc++/10132: filebuf destructor throws exceptions

[Benjamin Kosnik]

>This is disallowed by the resolution to DR 119. It won't
>fix the problem either since the destructor calls other
>functions that may throw (use_facet).

Right.

>I believe user-defined facets are allowed to throw anything
>they choose, whenever they choose from those members
>without throw specs, and that the iostream library should
>be prepared to deal with this.