[clang] [clang][ExprConst] Allow mutation in `__builtin_constant_p`'s argument (PR #159599)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

I agree with @shafik here, this needs more motivation.
I'm not sure we want to follow GCC's behavior here. 
In general `__builtin_constant_p` is pretty confusing and I would prefer to be conservative - or at least changing it might warrant a wider discussion / rfc

https://github.com/llvm/llvm-project/pull/159599