Overhaul CGU formation terminology.

Currently, the code uses multiple words to describe when a mono item `f`
uses a mono item `g`, all of which have problems.

- `f` references `g`: confusing because there are multiple kinds of use,
  e.g. "`f` calls `g`" is one, but "`f` takes a (`&T`-style) reference
  of `g`" is another, and that's two subtly different meanings of
  "reference" in play.

- `f` accesses `g`: meh, "accesses" makes me think of data, and this is
  code.

- `g` is a neighbor (or neighbour) of `f`: is verbose, and doesn't
  capture the directionality.

This commit changes the code to use "`f` uses `g`" everywhere. I think
it's better than the current terminology, and the consistency is
important.

Also, `InliningMap` is renamed `UsageMap` because (a) it was always
mostly about usage, and (b) the inlining information it did record was
removed in a recent commit.

Author: nnethercote