swift234

[Type checker] Be more rigorous about extracting argument labels from calls.

Whenever we have a call, retrieve the argument labels from the
argument structurally and associate them with the callee. We were
previously doing this as a separate AST walk (which was unnecessary),
so fold that into constraint generation for a CallExpr. We were also
allowing weird ASTs to effectively disable this information: tighten
that up and require that CallExprs always have a ParenExpr, TupleExpr,
or (as a temporary hack) a TypeExpr whose representation is a
TupleTypeRepr as their argument prior to type checking. This gives us
a more sane AST to work with, and guarantees that we aren't losing
label information.

From the user perspective, this should be NFC, because it's mostly AST
cleanup and staging.