Ensure an Rc isn't freed while running its own destructor.

A weak pointer inside itself will have its destructor run when the last
strong pointer to that data disappears, so we need to make sure that the
Weak and Rc destructors don't duplicate work (i.e. freeing).

By making the Rcs effectively take a weak pointer, we ensure that no
Weak destructor will free the pointer while still ensuring that Weak
pointers can't be upgraded to strong ones as the destructors run.

This approach of starting weak at 1 is what libstdc++ does.

Fixes #12046.
auto merge of #12047 : huonw/rust/cyclic-rc, r=thestinger

A weak pointer inside itself will have its destructor run when the last
strong pointer to that data disappears, so we need to make sure that the
Weak and Rc destructors don't duplicate work (i.e. freeing).

By making the Rcs effectively take a weak pointer, we ensure that no
Weak destructor will free the pointer while still ensuring that Weak
pointers can't be upgraded to strong ones as the destructors run.

This approach of starting weak at 1 is what libstdc++ does.

Fixes #12046.