[clang] [ClANG]Fix block pointer NTTP crash (PR #190377)

[Matheus Izvekov via cfe-commits]

mizvekov wrote:

I don't need the feature personally, and I can't ask anyone to implement it, but the option of allowing it where it's only usable with a null value is not well motivated.

So either:
* Disallow it.
* Implement it if you care about it working.

https://github.com/llvm/llvm-project/pull/190377