This does a few minor improvements:
1. Adds implicit conversions from NotNull to a raw pointer type if supported by
the underlying type, to make it so NotNull<RefPtr<T>> acts more like
RefPtr<T> in some situations.
2. Adds explicit conversion constructors and assignment operators for RefPtr
and nsCOMPtr from NotNull, avoiding conversion ambiguity added by the first
change.
3. Disable conversion constructors on NotNull with SFINAE if they should not be
available, meaning that type traits like std::is_convertible_v interact with
it properly.
Differential Revision: https://phabricator.services.mozilla.com/D168883
10582b6528