Pattern matching as currently implemented is consuming, but that's not
necessarily what we want to be the default behavior when borrowing pattern
matching is implemented. When a binding of noncopyable type is pattern-matched,
require it to be annotated with the `consume` operator explicitly. That way,
when we introduce borrowing pattern matching later, we have the option to make
`switch x` do the right thing without subtly changing the behavior of existing
code. rdar://110073984
previous checking for the expr being `self`
was checking for the decl being named `self`.
that meant you could do naughty things like:
```
let `self` = Self()
discard `self`
```
rdar://109376381
A bug was preventing you from writing `discard self`
in a consuming method of a generic noncopyable type.
The main cause was the `isPureMoveOnly` ignored
unbound generic types, claiming none of them are
noncopyable. Then I also needed to pass the contextual
type with the vars bound to the type checker.
rdar://108975216
SE-390 concluded with choosing the keyword discard rather than forget for
the statement that disables the deinit of a noncopyable type. This commit
adds parsing support for `discard self` and adds a deprecation warning for
`_forget self`.
rdar://108859077
