This resolves <https://github.com/swiftlang/sourcekit-lsp/issues/1788>,
following the discussion of alternatives on
<https://github.com/swiftlang/sourcekit-lsp/pulls/1789>. The bulk of the
change updates the translation from SourceKit placeholders to LSP
placeholders to handle nesting. The `CodeCompletionSession` also passes
a new custom formatter to the swift-syntax expansion routine, which
disables the transformation to trailing closures.
