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

[via cfe-commits]

Serosh-commits wrote:

@mizvekov  i think the  non-null test case will failwith `non-type template argument does not refer to any declaration`, so it seems block pointer NTTPs aren't fully supported beyond `nullptr` if u want i can implement  the full block pointer NTTP support but i am afraid it might be out of scope of what this pr does well let me know 

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