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

Author: redsun82

swift/extractor/infra/SwiftTagTraits.h

@@ -43,7 +43,7 @@ MAP(swift::Stmt, StmtTag)
   MAP(swift::BraceStmt, BraceStmtTag)
   MAP(swift::ReturnStmt, ReturnStmtTag)
   MAP(swift::YieldStmt, YieldStmtTag)
-  MAP(swift::ThenStmt, void)  // gated behind yet unusable experimental feature
+  MAP(swift::ThenStmt, ThenStmtTag)
   MAP(swift::DeferStmt, DeferStmtTag)
   MAP(swift::LabeledStmt, LabeledStmtTag)
     MAP(swift::LabeledConditionalStmt, LabeledConditionalStmtTag)

swift/extractor/translators/StmtTranslator.cpp

@@ -218,4 +218,10 @@ codeql::DiscardStmt StmtTranslator::translateDiscardStmt(const swift::DiscardStm
   return entry;
 }
 
+codeql::ThenStmt StmtTranslator::translateThenStmt(const swift::ThenStmt& stmt) {
+  auto entry = dispatcher.createEntry(stmt);
+  entry.result = dispatcher.fetchLabel(stmt.getResult());
+  return entry;
+}
+
 }  // namespace codeql

swift/extractor/translators/StmtTranslator.h

@@ -40,6 +40,7 @@ class StmtTranslator : public AstTranslatorBase<StmtTranslator> {
   codeql::FailStmt translateFailStmt(const swift::FailStmt& stmt);
   codeql::PoundAssertStmt translatePoundAssertStmt(const swift::PoundAssertStmt& stmt);
   codeql::DiscardStmt translateDiscardStmt(const swift::DiscardStmt& stmt);
+  codeql::ThenStmt translateThenStmt(const swift::ThenStmt& stmt);
 
  private:
   void fillLabeledStmt(const swift::LabeledStmt& stmt, codeql::LabeledStmt& entry);