[roll] Roll fuchsia [fidl][cpp] Don't use ToUnderlying when converting enums

When the new C++ bindings is used in Chromium we found that
HLCPP/natural enum conversion breaks when an enum-less library is
included in front of the library of interest. The reason is that we have
multiple overloads of `fidl::ToUnderlying`. There is one overload for
HLCPP strict enums, and separate overloads defined for natural strict &
flexble enums. Due to [1], a templated class that uses ToUnderlying can
only use ToUnderlying overloads that were already included before it.
Thus sometimes the natural strict & flexible overloads aren't visible to
the compiler.

In user code, people can call `fidl::ToUnderlying` and all the necessary
overloads are already included. However, inside FIDL library code such
as `lib/fidl/cpp/hlcpp_conversion.h`, not all `fidl::ToUnderlying` are
defined at the point when that header is included.

The most straightforward solution is to not use `fidl::ToUnderlying` in
FIDL runtime libraries.

[1]: https://en.cppreference.com/w/cpp/language/dependent_name

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/800049
Original-Revision: 4bbdd7f83187901fbf6bc2eb23b48d8749ef8dc9
GitOrigin-RevId: f3d44bbbe5560cea1c4e9b0f5eee63da6e20d337
Change-Id: I83cd227762b4a82bf04b814fce6fb83fae58aa9d