[clang] [Clang][Interp] Create the variable that holds the value of evaluating the binding (PR #100565)

[Timm Baeder via cfe-commits]

tbaederr wrote:

https://godbolt.org/z/a8TW93h63

Because it isn't a constant expression - at the time of the evaluation of the `static_assert`, the parameter value `cf` is unknown and so initializing `scf` can't work. Rejecting this is correct.

https://github.com/llvm/llvm-project/pull/100565