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.h

@@ -33,9 +33,6 @@ class ParseNodeKind {
   };
 
  public:
-  // TODO: See parse_node_kind.def.
-  static constexpr int32_t TodoFixParseNode = -2;
-
   // `clang-format` has a bug with spacing around `->` returns in macros. See
   // https://bugs.llvm.org/show_bug.cgi?id=48320 for details.
 #define CARBON_PARSE_NODE_KIND(Name)            \