modernize-use-if-consteval

Replaces direct std::is_constant_evaluated() checks in if statements with C++23’s if consteval syntax.

if (std::is_constant_evaluated())
  return slow_but_constexpr_path();
else
  return fast_runtime_path();

is rewritten as:

if consteval {
  return slow_but_constexpr_path();
} else {
  return fast_runtime_path();
}

The direct negated form is rewritten to if !consteval.