constexpr call depth counts calls whose arguments are still being evaluated #178

The constexpr depth count ends up being >2x what GCC counts.

Reproducer:

//options_all:--c++17 --max_depth_constexpr_call=300
constexpr int add(int a, int b) { return a + b; }
constexpr int f(int n) { return n == 0 ? 0 : add(1, f(n - 1)); }
static_assert(f(256) == 256, "");

Error:

error: expression must have a constant value
note: expression not folded to a constant due to excessive constexpr function call complexity

Reproducer should only have a depth of 257 instead of being rejected.

This impacts Boost.URL as some expressions are evaluating at depth >500 whereas G++ is ~250. Temporary workaround is to increase the call depth