Rather than setting an "Unwrap" field on the existing TemplateExpr, we'll
instead use a separate, simpler AST node.
There is very little in common between these two cases, so overloading the
TemplateExpr node doesn't buy much. A separate node is needed here,
rather than just returning the wrapped node directly, to give us somewhere
to capture the full source range extent of the wrapping template, whereas
the inner expression only captures the range of itself. This is important
both for good diagnostics and for transforming zclsyntax AST into
zclwrite AST.
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