Add initial support for parsing struct patterns (#7446)

Implements parsing of struct patterns as per
https://github.com/carbon-language/carbon-lang/issues/6680.

This handles the full and short syntax given in the [design
doc](https://github.com/carbon-language/carbon-lang/blob/trunk/docs/design/pattern_matching.md#struct-patterns),
but the handling of the shorthand syntax may need to change as I move on
to implementing Check support (currently the shorthand is represented as
one of `LetBindingPattern`, `VarBindingPattern`, or `VariablePattern`).

This implementation assumes that the answer to
https://github.com/carbon-language/carbon-lang/issues/7404 is that
trailing commas are allowed in the struct pattern, except for the case
where an `_` is present, in which case the next token must be the
closing brace `}`.

toolchain/check/handle_pattern_list.cpp

@@ -35,6 +35,11 @@ auto HandleParseNode(Context& context, Parse::TuplePatternStartId node_id)
   return HandlePatternListStart(context, node_id);
 }
 
+auto HandleParseNode(Context& context, Parse::StructPatternStartId node_id)
+    -> bool {
+  return context.TODO(node_id, "struct pattern start");
+}
+
 auto HandleParseNode(Context& context, Parse::ExplicitParamListStartId node_id)
     -> bool {
   context.full_pattern_stack().StartExplicitParamList();
@@ -129,6 +134,15 @@ auto HandleParseNode(Context& context, Parse::TuplePatternId node_id) -> bool {
   return true;
 }
 
+auto HandleParseNode(Context& context, Parse::StructPatternId node_id) -> bool {
+  return context.TODO(node_id, "struct pattern");
+}
+
+auto HandleParseNode(Context& context,
+                     Parse::StructPatternDesignatedFieldId node_id) -> bool {
+  return context.TODO(node_id, "struct pattern field");
+}
+
 auto HandleParseNode(Context& context, Parse::PatternListCommaId /*node_id*/)
     -> bool {
   EndExprRegionForPattern(context, context.node_stack());