Clean up a bunch of warnings in the tests, 3 of which actually turned out to be test bugs.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@131479 91177308-0d34-0410-b5e6-96231b3b80d8

test/utilities/utility/forward/move_only.pass.cpp

@@ -26,7 +26,7 @@ public:
 
 #ifndef _LIBCPP_HAS_NO_RVALUE_REFERENCES
     move_only(move_only&&) {}
-    move_only& operator=(move_only&&) {}
+    move_only& operator=(move_only&&) {return *this;}
 #else  // _LIBCPP_HAS_NO_RVALUE_REFERENCES
     operator std::__rv<move_only> () {return std::__rv<move_only>(*this);}
     move_only(std::__rv<move_only>) {}