Auto merge of #49836 - nikomatsakis:nll-facts-prep, r=pnkfelix

Auto merge of #49836 - nikomatsakis:nll-facts-prep, r=pnkfelix

prep work for using timely dataflow with NLL

Two major changes:

**Two-phase borrows are overhauled.** We no longer have two bits per borrow. Instead, we track -- for each borrow -- an (optional) "activation point". Then, for each point P where the borrow is in scope, we check where P falls relative to the activation point. If P is between the reservation point and the activation point, then this is the "reservation" phase of the borrow, else the borrow is considered active. This is simpler and means that the dataflow doesn't have to care about 2-phase at all, at last not yet.

**We no longer support using the MIR borrow checker without NLL.** It is going to be increasingly untenable to support lexical mode as we go forward, I think, and also of increasingly little value. This also exposed a few bugs in NLL mode due to increased testing.

r? @pnkfelix
cc @bobtwinkles