libstdc++: [_GLIBCXX_DEBUG] Simplify constant-evaluated checks

In the C++26 std::__debug::inplace_vector implementation we can use:

if consteval
{
  ...
}

rather than:

if (std::is_constant_evaluated())

libstdc++-v3/ChangeLog:

	* include/debug/inplace_vector
	(inplace_vector::emplace): Replace usages of std::is_constant_evaluated
	by consteval.
	(inplace_vector::insert): Likewise.
	(inplace_vector::erase): Likewise.