Swift: fix SequenceExpr extraction

[AlexDenisov]

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:

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(_:_:).cell@foo.swift: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).DatePickerCell.datePicker@foo.swift: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(_:_:).cell@foo.swift: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.

[MathiasVP]

One comment, but otherwise this LGTM

[MathiasVP]

For the sake of future developers looking at this piece of code: Does it maybe deserve an explanation for why we're only extracting the odd elements?

[AlexDenisov]

Right, I'll add a short comment and also a link to this PR (the PR description is in the commit message as well).

[MathiasVP]

LGTM!