[clang] [Clang] Diagnose forming references to nullptr (PR #143667)

[Shafik Yaghmour via cfe-commits]

================
@@ -35,6 +35,7 @@ enum AccessKinds {
   AK_Construct,
   AK_Destroy,
   AK_IsWithinLifetime,
+  AK_CheckReferenceInitialization
----------------
shafik wrote:

```suggestion
  AK_ReferenceInitialization
```

I could also go for `AK_ReadForReferenceInitialization`. The "Check" is what the code you are adding is doing for a read for reference initialization or at least that is how I see it.

https://github.com/llvm/llvm-project/pull/143667