[clang][nullability] allow _Nonnull etc on nullable class types (#82705)

This enables clang and external nullability checkers to make use of
these annotations on nullable C++ class types like unique_ptr.

These types are recognized by the presence of the _Nullable attribute.
Nullable standard library types implicitly receive this attribute.

Existing static warnings for raw pointers are extended to smart
pointers:

- nullptr used as return value or argument for non-null functions
  (`-Wnonnull`)
- assigning or initializing nonnull variables with nullable values
  (`-Wnullable-to-nonnull-conversion`)

It doesn't implicitly add these attributes based on the assume_nonnull
pragma, nor warn on missing attributes where the pragma would apply
them.
I'm not confident that the pragma's current behavior will work well for
C++ (where type-based metaprogramming is much more common than C/ObjC).
We'd like to revisit this once we have more implementation experience.

Support can be detected as `__has_feature(nullability_on_classes)`.
This is needed for back-compatibility, as previously clang would issue a
hard error when _Nullable appears on a smart pointer.

UBSan's `-fsanitize=nullability` will not check smart-pointer types.
It can be made to do so by synthesizing calls to `operator bool`, but
that's left for future work.

Discussion:
https://discourse.llvm.org/t/rfc-allowing-nonnull-etc-on-smart-pointers/77201/26

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