[mlir] ODS: require DefaultValuedAttr to be const-buildable

[ftynse]

Summary

ODS provides a mechanism for defalut-valued attributes based on a wrapper
TableGen class that is recognized by mlir-tblgen. Such attributes, if not set
on the operaiton, can be construted on-the-fly in their getter given a constant
value. In order for this construction to work, the attribute specificaiton in
ODS must set the constBuilderCall field correctly. This has not been verified,
which could lead to invalid C++ code being generated by mlir-tblgen.

Closes #53588.

[rriddle]

I'm assuming we don't have a great testing setup for this stuff, right? It's worth a bug to fix that.

[ftynse]

We don't have any negative testing for mlir-tblgen, I suspect it even fails assertions instead of reporting user-visible errors in some cases. Filed https://github.com/llvm/llvm-project/issues/53649.

mlir/tools/mlir-tblgen/OpDefinitionsGen.cpp

static void emitAttrGetterWithReturnType(FmtContext &fctx,
   auto *method = opClass.addMethod(attr.getReturnType(), name);
   ERROR_IF_PRUNED(method, name, op);
   auto &body = method->body();
   body << "  auto attr = " << name << "Attr();\n";
   if (attr.hasDefaultValue()) {
     // Returns the default value if not set.
     // TODO: this is inefficient, we are recreating the attribute for every
     // call. This should be set instead.
+    if (!attr.isConstBuildable()) {
+      PrintFatalError("DefaultValuedAttr of type " + attr.getAttrDefName() +
+                      " must have a constBuilder");
+    }
     std::string defaultValue = std::string(
         tgfmt(attr.getConstBuilderTemplate(), &fctx, attr.getDefaultValue()));
     body << "    if (!attr)\n      return "
          << tgfmt(attr.getConvertFromStorageCall(),
                   &fctx.withSelf(defaultValue))
          << ";\n";
   }
   body << "  return "