Fix a lowering crash when lowering a return by reference of a type with
an in-place initializing representation. We previously misinterpreted
this as an in-place initializing return.
This is addressed by changing lowering to interpret a `ReturnExpr` of a
reference expression as a reference return. However, that exposes
another issue: `return var;` produces a `ReturnExpr` of a reference
expression in the case where it returns in place! To fix that, we switch
`return var;` to producing a `ReturnExpr` of a value expression
regardless of whether the function has a return slot. This makes the
representation of `return var;` more uniform:
* If the expression is a reference, we're performing a `ref` return.
* If the expression is an initializing expression, we're performing a
normal by-initialization return.
* If the expression is a value expression, we're performing a `return
var;`.
David Blaikie
Richard Smith