[wgsl-reader] Allow decorations on function return types

Add a return type decoration list field to ast::Function.

Bug: tint:513
Change-Id: I41c1087f21a87731eb48ec7642997da5ae7f2baa
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/44601
Commit-Queue: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>

src/ast/function.cc

@@ -28,13 +28,15 @@ Function::Function(const Source& source,
                    VariableList params,
                    type::Type* return_type,
                    BlockStatement* body,
-                   DecorationList decorations)
+                   DecorationList decorations,
+                   DecorationList return_type_decorations)
     : Base(source),
       symbol_(symbol),
       params_(std::move(params)),
       return_type_(return_type),
       body_(body),
-      decorations_(std::move(decorations)) {
+      decorations_(std::move(decorations)),
+      return_type_decorations_(std::move(return_type_decorations)) {
   for (auto* param : params_) {
     TINT_ASSERT(param);
   }
@@ -77,7 +79,8 @@ Function* Function::Clone(CloneContext* ctx) const {
   auto* ret = ctx->Clone(return_type_);
   auto* b = ctx->Clone(body_);
   auto decos = ctx->Clone(decorations_);
-  return ctx->dst->create<Function>(src, sym, p, ret, b, decos);
+  auto ret_decos = ctx->Clone(return_type_decorations_);
+  return ctx->dst->create<Function>(src, sym, p, ret, b, decos, ret_decos);
 }
 
 void Function::to_str(const semantic::Info& sem,

src/ast/function.h

@@ -42,12 +42,14 @@ class Function : public Castable<Function, Node> {
   /// @param return_type the return type
   /// @param body the function body
   /// @param decorations the function decorations
+  /// @param return_type_decorations the return type decorations
   Function(const Source& source,
            Symbol symbol,
            VariableList params,
            type::Type* return_type,
            BlockStatement* body,
-           DecorationList decorations);
+           DecorationList decorations,
+           DecorationList return_type_decorations);
   /// Move constructor
   Function(Function&&);
 
@@ -74,6 +76,11 @@ class Function : public Castable<Function, Node> {
   /// @returns the function return type.
   type::Type* return_type() const { return return_type_; }
 
+  /// @returns the decorations attached to the function return type.
+  const DecorationList& return_type_decorations() const {
+    return return_type_decorations_;
+  }
+
   /// @returns a pointer to the last statement of the function or nullptr if
   // function is empty
   const Statement* get_last_statement() const;
@@ -108,6 +115,7 @@ class Function : public Castable<Function, Node> {
   type::Type* const return_type_;
   BlockStatement* const body_;
   DecorationList const decorations_;
+  DecorationList const return_type_decorations_;
 };
 
 /// A list of functions