[clang] [Clang] Fix P2564 handling of variable initializers (PR #89565)

[Mariya Podchishchaeva via cfe-commits]

Fznamznon wrote:

@katzdm 
It seems after this a `warn_impcast_integer_precision_constant` warning went missing:

https://godbolt.org/z/ndsPb44b4

Is that expected?

https://github.com/llvm/llvm-project/pull/89565