Copying a non-const FunctionRef lvalue should be a trivial operation

Copying a non-const FunctionRef lvalue should be a trivial operation

Summary: Before this change, when a non-const FunctionRef lvalue is copied, it is treated as any other callable: it is wrapped with an indirection. That's semantically incorrect and potentially creates lifetime problems. Instead, use the compiler generated copy constructor, which only copies the object and function pointers.

Reviewed By: yfeldblum

Differential Revision: D5040843

fbshipit-source-id: f691060bdced2e287ba22d22b961c02c2b924147