[clang] [clang] Remove an incorrect assertion in ConstantFoldAttrs (PR #105789)

[Timm Baeder via cfe-commits]

tbaederr wrote:

I guess it might make sense to create the `ConstantExpr` in any case, even for a `None` `APValue`? Dumping the `ConstantExpr` didn't print any `APValue` though, so I was assuming that it ignores `None` anyway.

https://github.com/llvm/llvm-project/pull/105789