Defining it as a `DECL_KEYWORD` means code
completion will try and complete it in decl
position. It shouldn't really be considered a decl
keyword, so define it as `SWIFT_KEYWORD`.
These allow multi-statement `if`/`switch` expression
branches that can produce a value at the end by
saying `then <expr>`. This is gated behind
`-enable-experimental-feature ThenStatements`
pending evolution discussion.
