Since the desugaring removed special handling for ranges, the error
message changed and so I had to adjust `range-1`.
Turns out there was a bug where borrowck was too restrictive in some
rare cases of constructing ranges from literals. The `range-2` test
enshrined this bug -- now it's adjusted to test a case that's actually
wrong.
Allows the expression `..` (without either endpoint) in general, can be
used in slicing syntax `&expr[..]` where we previously wrote `&expr[]`.
The old syntax &expr[] is not yet removed or warned for.
