The move / swap members were not correctly taking all of the possible states of the basic_stringbuf into account.  Just rewrote these members.  Test included.  This fixes http://llvm.org/bugs/show_bug.cgi?id=15659.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@178690 91177308-0d34-0410-b5e6-96231b3b80d8