SE-0362: Updates based on review & language workgroup discussions (#1705)

* SE-0362: Adopt Ben Rimmington's SwiftPM manifest API

* Use the term "upcoming feature" rather than "future feature" to reduce confusion.

* Don't parse the right-hand side of a `&&` or `||` that doesn't affect the result.

* Add some discussion of language and tools versions.

* Extend the SE-0362 review to July 11th.

Co-authored-by: Holly Borla <[email protected]>

Author: DougGregor