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_unused.cpp

@@ -19,8 +19,9 @@ auto HandleUnusedPattern(Context& context) -> void {
   context.ConsumeChecked(Lex::TokenKind::Unused);
 
   context.PushStateForPattern(StateKind::Pattern, state.in_var_pattern,
-                              /*in_unused_pattern=*/true, state.binding_context,
-                              state.ambient_precedence);
+                              /*in_unused_pattern=*/true,
+                              state.in_field_shorthand_pattern,
+                              state.binding_context, state.ambient_precedence);
 }
 
 auto HandleFinishUnusedPattern(Context& context) -> void {