[flang] Improvements suggested by Tim.

Original-commit: flang-compiler/f18@c3d2e3e745
Reviewed-on: https://github.com/flang-compiler/f18/pull/20
Tree-same-pre-rewrite: false

flang/documentation/C++style.md

@@ -77,6 +77,11 @@ the end of the line, not on the next line.  Functions also put the opening
 `{` after the formal arguments or new-style result type, not on the next
 line.  Use `{}` for empty inline constructors and destructors in classes.
 
+If any branch of an `if`/`else if`/`else` cascade ends with a return statement,
+they all should, with the understanding that the cases are all unexceptional.
+When testing for an error case that should cause an early return, do so with
+an `if` that doesn't have a following `else`.
+
 Don't waste space on the screen with needless blank lines or elaborate block
 commentary (lines of dashes, boxes of asterisks, &c.).  Write code so as to be
 easily read and understood with a minimum of scrolling.

flang/lib/parser/idioms.h

@@ -131,6 +131,7 @@ std::ostream &formatTuple(std::ostream &o, const T &x) {
   if constexpr (J < std::tuple_size_v<T>) {
     return formatTuple<J + 1>(o << std::get<J>(x), x);
   }
+  return o;
 }
 
 template<typename... As>

flang/lib/parser/parse-tree-visitor.h

@@ -73,11 +73,21 @@ Walk(const A &x, V &visitor) {
 
 template<typename A, typename V>
 typename std::enable_if<TupleTrait<A>>::type
-Walk(const A &x, V &visitor) { Walk(x.t, visitor); }
+Walk(const A &x, V &visitor) {
+  if (visitor.Pre(x)) {
+    Walk(x.t, visitor);
+    visitor.Post(x);
+  }
+}
 
 template<typename A, typename V>
 typename std::enable_if<UnionTrait<A>>::type
-Walk(const A &x, V &visitor) { Walk(x.u, visitor); }
+Walk(const A &x, V &visitor) {
+  if (visitor.Pre(x)) {
+    Walk(x.u, visitor);
+    visitor.Post(x);
+  }
+}
 
 template<typename A, typename V>
 typename std::enable_if<WrapperTrait<A>>::type