…varId`.

(This refactoring looks like a wash in this commit, in terms of
overall code-cleanliness; but it makes things nicer in a future commit
for static drop semantics.)

…ef and by-value captures.

This is necessary to be able to properly backtrack in a loan-path to
the definition site of the original variable (which I found necessary
in later commits).

`LpDowncast` carries the `DefId` of the variant itself.  To support
this, added the enum variant `DefId` to the `cat_downcast` variant in
`mem_categorization::categorization`.

To make this clean, refactored old `LoanPath` enum into a
`LoanPath` struct with a `ty::t` and a newly-added `LoanPathVariant` enum.

This enabled me to get rid of the ugly and fragile `LoanPath::to_type`
method, and I can probably also get rid of other stuff that was
supporting it, maybe.

…ost move.

This is accomplished by:

1. Add `MatchMode` enum to `expr_use_visitor`.

2. Computing the match mode for each pattern via a pre-pass, and then
   passing the mode along when visiting the pattern in
   expr_use_visitor.

3. Adding a `fn matched_pat` callback to expr_use_visitor, which is called on
   all nodes of the pattern (as opposed to `fn consume_pat`, which is only
   invoked for identifiers at the leaves of the pattern), and invoking it
   accordingly.

Of particular interest are the `cat_downcast` instances established
when matching enum variants.

Note that due to PR rust-lang#16053, we only actually deal with `a @ Var(_)`
here, (as opposed to a hypothetical `a @ Var(ref b)`, which cannot
currenty arise, and maybe will never be supported for non-copy data).

…puts.

…m output.

… a span.

This will be utilized by the rustc_drop_obligations
code-annotation-via-errors hack to print out the drop obligations that
are removed at the end of scopes.

When you add `rustc_drop_obligations` as an annotation on a function,
it will print all of the drop-obligation manipulations as individual
compiler errors associated with the expressions that caused the effect
on the drop obligation state.

Surround drop-obligations in output with `` quotes to help unintended prefix matches

Prints scope-end messages at end of their scope.

…bute.