Also stop supporting `var` with initializer inside `for`.
Resolves TODO in `handle_variable.cpp`
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Co-authored-by: Richard Smith <richard@metafoo.co.uk>
(Split out of #3410)
Consistently use `ParenExpr` solely for parenthesized single
expressions, and use more syntax-oriented terminology for states and
nodes that might represent either a `ParenExpr` or a tuple literal.