PR c++/93324 - ICE with -Wall on constexpr if.

PR c++/93324 - ICE with -Wall on constexpr if.

This is a crash with constexpr if, when trying to see if the call in
the if-statement is std::is_constant_evaluated.

cp_get_callee_fndecl_nofold can return NULL_TREE and fndecl_built_in_p
doesn't expect to get a null tree, so check FNDECL first.