Swift: fix SequenceExpr extraction

Before we extracted all the subexpressions from the `SequenceExpr` while we should've only extracted the expressions at odd indices:
```
...
/// SequenceExpr - A list of binary operations which has not yet been
/// folded into a tree.  The operands all have even indices, while the
/// subexpressions with odd indices are all (potentially overloaded)
/// references to binary operators.
class SequenceExpr final : public Expr,
...
```

The AST for a `SequenceExpr` looks like this:

```
sequence_expr:
  unresolved_dot_expr:
    ...
  assign_expr:
    member_ref_expr:
      ...
    dot_syntax_call_expr:
      ...
  unresolved_member_chain_expr:
    ...
```

however, what's is not visible with the "final" AST is that `unresolved_dot_expr` is the unresolved version of `assign_expr.member_ref_expr` and the `unresolved_member_chain_expr` is the unresolved version of `assign_expr.dot_syntax_call_expr`.
This becomes visible when I enable typechecker debugging:

```c++
auto &typeCheckerOptions = invocation.getTypeCheckerOptions();
typeCheckerOptions.DebugConstraintSolver = true;
```

Which prints the following snippets:

```
---Initial constraints for the given expression---
(assign_expr type='()' location=foo.swift:25:54 range=[foo.swift:25:13 - line:25:57]
  (unresolved_dot_expr type='$T2' location=foo.swift:25:29 range=[foo.swift:25:13 - line:25:29] field 'preferredDatePickerStyle' function_ref=unapplied
    (unresolved_dot_expr type='$T1' location=foo.swift:25:18 range=[foo.swift:25:13 - line:25:18] field 'datePicker' function_ref=unapplied
      (declref_expr type='DatePickerCell' location=foo.swift:25:13 range=[foo.swift:25:13 - line:25:13] decl=foo.(file).DatePickerRowProtocol extension.configurePickerStyle(_:_:)[email protected]:15:33 function_ref=unapplied)))
  (unresolved_member_chain_expr implicit type='$T5' location=foo.swift:25:57 range=[foo.swift:25:56 - line:25:57]
    (unresolved_member_expr type='$T4' location=foo.swift:25:57 range=[foo.swift:25:56 - line:25:57] name='wheels' function_ref=unapplied)))

// ...

---Type-checked expression---
(assign_expr type='()' location=foo.swift:25:54 range=[foo.swift:25:13 - line:25:57]
  (member_ref_expr type='@lvalue UIDatePickerStyle' location=foo.swift:25:29 range=[foo.swift:25:13 - line:25:29] decl=UIKit.(file).UIDatePicker.preferredDatePickerStyle
    (force_value_expr implicit type='UIDatePicker' location=foo.swift:25:18 range=[foo.swift:25:13 - line:25:18] implicit_iuo_unwrap
      (load_expr implicit type='UIDatePicker?' location=foo.swift:25:18 range=[foo.swift:25:13 - line:25:18]
        (member_ref_expr type='@lvalue UIDatePicker?' location=foo.swift:25:18 range=[foo.swift:25:13 - line:25:18] decl=foo.(file)[email protected]:10:29
          (declref_expr type='DatePickerCell' location=foo.swift:25:13 range=[foo.swift:25:13 - line:25:13] decl=foo.(file).DatePickerRowProtocol extension.configurePickerStyle(_:_:)[email protected]:15:33 function_ref=unapplied)))))
  (dot_syntax_call_expr type='UIDatePickerStyle' location=foo.swift:25:57 range=[foo.swift:25:56 - line:25:57]
    (declref_expr type='(UIDatePickerStyle.Type) -> UIDatePickerStyle' location=foo.swift:25:57 range=[foo.swift:25:57 - line:25:57] decl=UIKit.(file).UIDatePickerStyle.wheels function_ref=unapplied)
    (argument_list implicit
      (argument
        (type_expr implicit type='UIDatePickerStyle.Type' location=foo.swift:25:56 range=[foo.swift:25:56 - line:25:56] typerepr='UIDatePickerStyle')))))
```

The proposed solution is to only extract subexpressions at indices from `SequenceExpr` thus ignoring all the unresolved leftovers.

Note: I'm not entirely sure about the case when there is only child (`elements.size() == 1`) so I'm always extracting it.

This patch fixes the last source of unresolved expressions.

Author: AlexDenisov

swift/extractor/translators/ExprTranslator.cpp

@@ -460,7 +460,17 @@ codeql::UnresolvedMemberExpr ExprTranslator::translateUnresolvedMemberExpr(
 
 codeql::SequenceExpr ExprTranslator::translateSequenceExpr(const swift::SequenceExpr& expr) {
   auto entry = createExprEntry(expr);
-  entry.elements = dispatcher.fetchRepeatedLabels(expr.getElements());
+  // SequenceExpr represents a flat tree of expressions with elements at odd indices being the
+  // parents of the elements with even indices, so we only extract the "parent" elements here. In
+  // case there is a single child, we extract it as a parent. See
+  // https://github.com/github/codeql/pull/14119 and commit message for more details.
+  if (expr.getNumElements() == 1) {
+    entry.elements = dispatcher.fetchRepeatedLabels(expr.getElements());
+  } else {
+    for (int i = 1; i < expr.getNumElements(); i += 2) {
+      entry.elements.emplace_back(dispatcher.fetchLabel(expr.getElement(i)));
+    }
+  }
   return entry;
 }