[clang] [clang][Sema] Reject negative tuple sizes (PR #159579)

[Timm Baeder via cfe-commits]

tbaederr wrote:

`UnsignedOrNone` just means we assert there when passing `-1`. With a full `std::optional<unsigned>` the value would just underflow and the value we use later would've been wrong. The incorrect handling of negative values has nothing to do with `UnsignedOrNone`.

https://github.com/llvm/llvm-project/pull/159579