C++: Improve constexpr call hashing and garbage collection.

This commit addresses a use-after-free issue that could occur during the evaluation of recursive constexpr lambdas, particularly when the -Wtautological-compare flag is enabled. The fix involves unsharing bindings earlier in the cxx_eval_call_expression function, specifically when creating the entry in the constexpr_call_table. This ensures that the entry refers to valid memory, even after garbage collection.