ProgramBuilder: Tweak Return() signature

Instead of directly fowarding to ReturnStatement, pass the parameter through `Expr()`.
Also add a no-arg overload.

This means we can write `Return(1);`, `Return("a")`, `Return()`

Change-Id: I8f5df630f540f9cecdf82d24e2810a0844e025e8
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/48046
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: James Price <jrprice@google.com>

src/program_builder.h

@@ -1149,12 +1149,16 @@ class ProgramBuilder {
     return func;
   }
 
-  /// Creates an ast::ReturnStatement with the input args
-  /// @param args arguments to construct a return statement with
+  /// Creates an ast::ReturnStatement with no return value
   /// @returns the return statement pointer
-  template <typename... Args>
-  ast::ReturnStatement* Return(Args&&... args) {
-    return create<ast::ReturnStatement>(std::forward<Args>(args)...);
+  ast::ReturnStatement* Return() { return create<ast::ReturnStatement>(); }
+
+  /// Creates an ast::ReturnStatement with the given return value
+  /// @param val the return value
+  /// @returns the return statement pointer
+  template <typename EXPR>
+  ast::ReturnStatement* Return(EXPR&& val) {
+    return create<ast::ReturnStatement>(Expr(std::forward<EXPR>(val)));
   }
 
   /// Creates a ast::Struct and type::Struct, registering the type::Struct with