Introduce then statements

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.

lib/SILOptimizer/Mandatory/MoveOnlyDiagnostics.cpp

@@ -246,6 +246,7 @@ void DiagnosticEmitter::emitMissingConsumeInDiscardingContext(
       case StmtKind::Return:
       case StmtKind::Yield:
       case StmtKind::Break:
+      case StmtKind::Then:
       case StmtKind::Fail:
       case StmtKind::PoundAssert:
         return true;