Non-recursive variant of detail::length() for compilers with maximum recursion depth in compile-time constexpr evaluation.

[AdmiralCurtiss]

I ran into this in C++14 when creating a string_view with a very long string. For example,

namespace {
    constexpr ::nonstd::string_view tmp = ::nonstd::string_view(
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
        "01234567890123456789012345678901234567890123456789"
    );
}

On MSVC 2019 in C++14 mode this gives:

error C2131: expression did not evaluate to a constant
  note: failure was caused by evaluation exceeding call depth limit of 512 (/constexpr:depth<NUMBER>)

And on g++ 9.2.1 this gives

error: ‘constexpr’ evaluation depth exceeds maximum of 512 (use ‘-fconstexpr-depth=’ to increase the maximum)

I don't think we can fix this for C++11, but when we have C++14 we have better ways to write a constexpr length() of a nullterminated string, so might as well do so to work around this problem.