Swift: add extraction of ThenStmt

These are currently added implicitly by the compiler in the context of
`if`/`switch` expressions. In the future, there might be explicit
`then <expr>` statement useful for cases where one would like to add
more than one statement in the branch, to mark what value to actually
use.

See https://forums.swift.org/t/pitch-multi-statement-if-switch-do-expressions/68443

swift/schema.py

@@ -994,6 +994,20 @@ class ThrowStmt(Stmt):
 class YieldStmt(Stmt):
     results: list[Expr] | child
 
+@qltest.test_with('SingleValueStmtExpr')
+class ThenStmt(Stmt):
+    """ A statement implicitly wrapping values to be used in branches of if/switch expressions. For example in:
+    ```
+    let rank = switch value {
+        case 0..<0x80: 1
+        case 0x80..<0x0800: 2
+        default: 3
+    }
+    ```
+    the literal expressions `1`, `2` and `3` are wrapped in `ThenStmt`.
+    """
+    result: Expr
+
 class DoCatchStmt(LabeledStmt):
     body: Stmt | child
     catches: list[CaseStmt] | child