There are a number of implicit conversions in Swift, such as to Optional
and to an existential, which are now possible for noncopyable types.
But all type casts are consuming operations for noncopyable types. So
it's confusing when a function that takes a borrowed argument of
optional type appears to be consuming:
```
func f(_ x: borrowing NC?) { ... }
let x = NC()
f(x)
f(x) // error!
```
So, rather than for people to write `x as T?` around all implicit
conversions, require them to write `consume x` around expressions
that will consume some lvalue. Since that makes it much more clear what
the consequences will be.
Expressions like `f(g())`, where you're passing an rvalue to the callee,
are not confusing. And those are exactly the expressions you're not
allowed to write `consume` for, anyway.
fixes rdar://127450418
