wgsl: Deprecate '-> void' on functions

Was removed with:
https://github.com/gpuweb/gpuweb/pull/1460

Bug: tint:677
Change-Id: If08cb450189c6158561051ef6e8f2439c60bc010
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/47140
Auto-Submit: Ben Clayton <bclayton@google.com>
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: David Neto <dneto@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/src/reader/wgsl/parser_impl.h b/src/reader/wgsl/parser_impl.h
index c048a3e..7f4929b 100644
--- a/src/reader/wgsl/parser_impl.h
+++ b/src/reader/wgsl/parser_impl.h
@@ -320,7 +320,11 @@
   /// @return `Failure::Errored::kError` so that you can combine an add_error()
   /// call and return on the same line.
   Failure::Errored add_error(const Source& source, const std::string& msg);
-
+  /// Appends a deprecated-language-feature warning at `source` with the message
+  /// `msg`
+  /// @param source the source to associate the error with
+  /// @param msg the warning message
+  void deprecated(const Source& source, const std::string& msg);
   /// Registers a constructed type into the parser
   /// @param name the constructed name
   /// @param type the constructed type