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/testdata/generics/interface/fail_missing_name.carbon

@@ -5,9 +5,10 @@
 // AUTOUPDATE
 // RUN: %{not} %{carbon-run-parser}
 // CHECK:STDOUT: [
+// CHECK:STDOUT:   {kind: 'DeclaredName', text: 'interface', has_error: yes},
 // CHECK:STDOUT:     {kind: 'InterfaceBodyStart', text: '{'},
 // CHECK:STDOUT:   {kind: 'InterfaceBody', text: '}', subtree_size: 2},
-// CHECK:STDOUT: {kind: 'InterfaceDefinition', text: 'interface', has_error: yes, subtree_size: 3},
+// CHECK:STDOUT: {kind: 'InterfaceDefinition', text: 'interface', has_error: yes, subtree_size: 4},
 // CHECK:STDOUT: {kind: 'FileEnd', text: ''},
 // CHECK:STDOUT: ]