Cannot disable formatting around a field

[dplassgit]

Describe the bug
If you try to disable formatting around a "type" the formatter crashes.

To Reproduce
Steps to reproduce the behavior:
Try to format this:

struct Point {
    // dslx-fmt::off
    x: u32,    
    // dslx-fmt::on
    y: u32,
}

It segfaults:

F1202 08:05:30.335632    4899 logging.cc:62] assert.h assertion failed at third_party/xls/common/casts.h:59 in To xls::down_cast(From *) [To = xls::dslx::TypeAnnotation *, From = xls::dslx::AstNode]: f == nullptr || dynamic_cast<To>(f) != nullptr
*** Check failure stack trace: ***
    @     0x7f84ca3cd399  absl::log_internal::LogMessage::SendToLog()
    @     0x7f84ca3cc8ae  absl::log_internal::LogMessage::Flush()
    @     0x7f84ca3cda19  absl::log_internal::LogMessageFatal::~LogMessageFatal()
    @     0x7f84db999574  __assert_fail
    @     0x7f8503270f63  xls::dslx::(anonymous namespace)::AstCloner::HandleStructDef()

Basically it's because VerbatimNode doesn't implement TypeAnnotation.

Expected behavior
Output = input

[dplassgit]

This happens in other places too, e.g. in function parameters.

[dplassgit]

Seems like StructMembers aren't AST nodes so we can't completely replace them with VerbatimNodes.