Add a clang plugin check that flags GC::Cell subclasses (and their
base classes within the Cell hierarchy) that have destructors with
non-trivial bodies. Such logic should use Cell::finalize() instead.
Add GC_ALLOW_CELL_DESTRUCTOR annotation macro for opting out in
exceptional cases (currently only JS::Object).
This prevents us from accidentally adding code in destructors that
runs after something we're pointing to may have been destroyed.
(This could become a problem when the garbage collector sweeps
objects in an unfortunate order.)
This new check uncovered a handful of bugs which are then also fixed
in this commit. :^)
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
