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/parse/handle_statement.cpp

@@ -118,10 +118,10 @@ auto HandleStatementForHeader(Context& context) -> void {
 
   state.kind = StateKind::StatementForHeaderIn;
   context.PushState(state);
-  context.PushStateForPattern(StateKind::Pattern, /*in_var_pattern=*/false,
-                              /*in_unused_pattern=*/false,
-                              BindingContext::ExplicitParam,
-                              PrecedenceGroup::ForTopLevelPattern());
+  context.PushStateForPattern(
+      StateKind::Pattern, /*in_var_pattern=*/false,
+      /*in_unused_pattern=*/false, /*in_field_shorthand_pattern=*/false,
+      BindingContext::ExplicitParam, PrecedenceGroup::ForTopLevelPattern());
 }
 
 auto HandleStatementForHeaderIn(Context& context) -> void {