Update synposis in <memory> to show move semantics for weak_ptr; add tests for...

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.

llvm-svn: 202931