Remove `GCX_PTR`.
We store an `ImplicitCtxt` pointer in a thread-local value (TLV). This allows
implicit access to a `GlobalCtxt` and some other things.
We also store a `GlobalCtxt` pointer in `GCX_PTR`. This is always the same
`GlobalCtxt` as the one within the `ImplicitCtxt` pointer in TLV. `GCX_PTR`
is only used in the parallel compiler's `handle_deadlock()` function.
This commit does the following.
- It removes `GCX_PTR`.
- It also adds `ImplicitCtxt::new()`, which constructs an `ImplicitCtxt` from a
`GlobalCtxt`. `ImplicitCtxt::new()` + `tls::enter_context()` is now
equivalent to the old `tls::enter_global()`.
- Makes `tls::get_tlv()` public for the parallel compiler, because it's
now used in `handle_deadlock()`.
r? @petrochenkov