swift234

Add some availability attributes for UnsafePointer conversion.

This introduces some important diagnostics to help migration.

Some of the diagnostics that we would like to provide are impossible
on a generic class because they introduce ambiguity.

Renaming UnsafePointer<Void> to UnsafeRawPointer makes extensions and
global operators ambiguous.

We would also like to provide this fix-it on conversion from
UnsafePointer<Void> or UnsafeRawPointer to UnsafePointer<T>
with this message:
  Conversion restricted. Use 'assumingMemoryBound(to:)' or 'bindMemory(to:capacity:)'}}

However, that introduces ambiguous overloads that defeat other hueristics.