Add case_selectors update

This CL adds the missing case_selectors option from the grammar updates.

Change-Id: Ia6c110e917dd574711d396fb34ad53a2a67cf1fe
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/22306
Reviewed-by: David Neto <dneto@google.com>
diff --git a/src/reader/wgsl/parser_impl.h b/src/reader/wgsl/parser_impl.h
index 826ea8a..718f3c2 100644
--- a/src/reader/wgsl/parser_impl.h
+++ b/src/reader/wgsl/parser_impl.h
@@ -23,6 +23,7 @@
 
 #include "src/ast/assignment_statement.h"
 #include "src/ast/builtin.h"
+#include "src/ast/case_statement.h"
 #include "src/ast/constructor_expression.h"
 #include "src/ast/else_statement.h"
 #include "src/ast/entry_point.h"
@@ -216,6 +217,9 @@
   /// Parses a `switch_body` grammar element
   /// @returns the parsed statement or nullptr
   std::unique_ptr<ast::CaseStatement> switch_body();
+  /// Parses a `case_selectors` grammar element
+  /// @returns the list of literals
+  ast::CaseSelectorList case_selectors();
   /// Parses a `case_body` grammar element
   /// @returns the parsed statements
   ast::StatementList case_body();