When trying to disable formatting around a test function it segfaults

[dplassgit]

To Reproduce
Try to format:

#[test]
// dslx-fmt::off
fn test() {
}
// dslx-fmt::on

Expected behavior
Output = input

Actual behavior

segfault:

F1203 08:52:26.712443    8951 logging.cc:62] assert.h assertion failed at third_party/xls/common/casts.h:59 in To xls::down_cast(From *) [To = xls::dslx::Function *, From = xls::dslx::AstNode]: f == nullptr || dynamic_cast<To>(f) != nullptr
*** Check failure stack trace: ***
...
    @     0x7ff4f146c800  xls::dslx::(anonymous namespace)::AstCloner::HandleTestFunction()
    @     0x7ff4f147a65d  xls::dslx::(anonymous namespace)::AstCloner::ReplaceOrVisit()
    @     0x7ff4f14858cb  std::__u::__variant_detail::__visitation::__base::__dispatcher<>::__dispatch<>()
    @     0x7ff4f1468b7c  xls::dslx::(anonymous namespace)::AstCloner::HandleModule()
    @     0x7ff4f1465b92  xls::dslx::CloneModule()
    @     0x7ff55f70fbc5  xls::dslx::AutoFmt()
    @     0x7ff55fb861cf  xls::dslx::(anonymous namespace)::ModuleFmtTest::DoFmt()
...

[dplassgit]

This is partially because it can't find the comment between the #[test] and the function, and partially because the NameDef isn't castable to a VerbatimNode. We'll disallow format disabling in this situation.