Nullable flow analysis for unmanaged pointers

## Problem / Motivation

Roslyn does nullable flow analysis to warn when a reference type might be null. However, unmanaged pointers (e.g. `int*`, `void*`, etc.) currently do *not* participate in nullable flow analysis, so potential null dereferences can not be reported in the same way.

This gap can lead to safety issues in unsafe code, and makes it harder for developers to reason about pointer nullability. It also reduces consistency, reference type nullability is tracked, but pointer nullability is not.

## Proposal

- Extend Roslyn’s nullable flow analysis to include unmanaged pointers.

## Related Issues
* https://github.com/dotnet/runtime/pull/119745

Start by reading Roslyn's existing nullable flow analysis and the related dotnet/runtime pull request linked in the issue. The work is done when unmanaged pointers such as int* and void* participate in nullable flow analysis and potential null dereferences receive corresponding warnings.

Written by the indexing model from the issue text.