I've been thinking about this since we decided to add placeholders in
the parse tree. This allows a clearer division of work in check
handling, where we were doing work for ExprOpenParen that's only
necessary for tuples (splitting/renaming handle_paren.cpp accordingly).
(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.
Jon Ross-Perkins
josh11bandJosh L
Geoff Romer