Allow `auto` as a return type for functions. Only support functions with one `return` statement for now (open question).
This explicitly suggests removing the executable semantics `fn name(args) => expression` syntax, and is motivated by reconciling executable semantics with approved Carbon state. [example](https://github.com/carbon-language/carbon-lang/blob/3d1716f6c692a840d8b4b513ddfb8119432a5150/executable_semantics/testdata/fun_named_params.carbon) This aspect is a decision that may be affected by lambda syntax, but we might also choose to keep lambda syntax and function syntax separate -- I don't think there's enough benefit to providing the alternate function syntax right now.
Co-authored-by: Geoff Romer <gromer@google.com>
Co-authored-by: Chandler Carruth <chandlerc@gmail.com>
