Finish bracketing of parse nodes. (#2430)

- Finishes remaining "todo" parse nodes.
- Improving error recovery for invalid designators and structs, so that the parse tree still looks similar to a valid parse tree.
- Call expressions now have the thing being called as a child (of the start) instead of a sibling.
- Use of Start is replacing use of End in several parse nodes, like structs and call expressions.
- Adjusting documentation of parse node structures in an attempt to make it more consistent and understandable.
- The current state for interfaces and if/else is mostly being documented, not altered.

toolchain/parser/parse_node_kind.cpp

@@ -47,8 +47,7 @@ auto ParseNodeKind::child_count() const -> int32_t {
 #include "toolchain/parser/parse_node_kind.def"
   };
   auto child_count = ChildCount[static_cast<int>(kind_)];
-  // TODO: Allows TodoFixParseNode; see parse_node_kind.def.
-  CARBON_CHECK(child_count != -1) << *this;
+  CARBON_CHECK(child_count >= 0) << *this;
   return child_count;
 }