Add -Wpessimizing-move and -Wredundant-move warnings.

-Wpessimizing-move warns when a call to std::move would prevent copy elision
if the argument was not wrapped in a call.  This happens when moving a local
variable in a return statement when the variable is the same type as the
return type or using a move to create a new object from a temporary object.

-Wredundant-move warns when an implicit move would already be made, so the
std::move call is not needed, such as when moving a local variable in a return
that is different from the return type.

Differential Revision: http://reviews.llvm.org/D7633

llvm-svn: 236075