[Bug libstdc++/54316] [C++11] move constructor for stringstream

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54316
Bug 54316 depends on bug 53626, which changed state.

Bug 53626 Summary: [C++11] move assignment for ifstream
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53626

           What    |Removed                     |Added
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             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED