Update synposis in <memory> to show move semantics for weak_ptr; add tests for already existing move semantics. Mark LWG issues #2315 (no changes needed), 2316 (move semantics for weak_ptr), 2252 (previous commit) and 2271 (previous commit) as complete.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@202931 91177308-0d34-0410-b5e6-96231b3b80d8