GCOVProfiling: Avoid use-after-move

Turns out this was use-after-move of function_ref, which is trivially
copyable and movable, so the move did nothing and use after move was
safe.

But since this function_ref is being copied into a std::function, change
the function_ref to be std::function to avoid extra layers of type
erasure indirection - and then it's a real use after move, and fix that
by referring to the moved-to member variable rather than the moved-from
parameter.