[mlir][ods] Emit bytecode readers with generic Attribute/Type (NFC)

This aligns the API with the builder API that allows arbitrary
attributes or types to be returned.

Author: Mogball

mlir/test/mlir-tblgen/bytecode-reserved.td

@@ -2,6 +2,8 @@
 
 include "mlir/IR/BuiltinDialectBytecode.td"
 
+// CHECK static ::mlir::Type readIntegerType
+
 def TestDialectTypes : DialectTypes<"Test"> {
 // CHECK: static Type readType
   let elems = [

mlir/tools/mlir-tblgen/BytecodeDialectGen.cpp

@@ -134,7 +134,7 @@ void Generator::emitParse(StringRef kind, Record &x) {
       R"(static {0} read{1}(MLIRContext* context, DialectBytecodeReader &reader) )";
   mlir::raw_indented_ostream os(output);
   std::string returnType = getCType(&x);
-  os << formatv(head, returnType, x.getName());
+  os << formatv(head, kind == "attribute" ? "::mlir::Attribute" : "::mlir::Type", x.getName());
   DagInit *members = x.getValueAsDag("members");
   SmallVector<std::string> argNames =
       llvm::to_vector(map_range(members->getArgNames(), [](StringInit *init) {