Fix uses of __cpp_lib_constexpr_string

[keith]

The intent of these checks was that if the C++20 version of this feature are enabled to execute the top codepath. libstdc++ 12 defines this feature as:

// in: /usr/include/c++/12/bits/basic_string.h

#ifdef __cpp_lib_is_constant_evaluated
// Support P0980R1 in C++20.
# define __cpp_lib_constexpr_string 201907L
#elif __cplusplus >= 201703L && _GLIBCXX_HAVE_IS_CONSTANT_EVALUATED
// Support P0426R1 changes to char_traits in C++17.
# define __cpp_lib_constexpr_string 201611L
#endif

So this codepath was always being hit even with -std=c++17 and then resulted in a failure because the string wasn't actually constinit. This matches the other use of this feature check in inlined_string_field.h