[Bug c++/110158] Cannot use union with std::string inside in constant expression

["danakj at orodu dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110158

--- Comment #4 from danakj at orodu dot net ---
Here's a repro without the std::string inside a union. It is the SSO union
inside the string that causes the error.

https://gcc.godbolt.org/z/T8oM8vYnq

```
#include <string>

template <class T, class I, class S, class F>
constexpr T fold(T init, I i, S s, F f) {
    while (true) {
        if (i == s)
            return init;
        else
            init = f(std::move(init), *i++);
    }
}

constexpr char v[] = {'a', 'b', 'c'};
static_assert(fold(std::string(), std::begin(v), std::end(v),
                   [](std::string acc, char v) {
                       acc.push_back(v);
                       return acc;
                   }) == "abc");

int main() {}
```

<source>:18:23: error: non-constant condition for static assertion
   14 | static_assert(fold(std::string(), std::begin(v), std::end(v),
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   15 |                    [](std::string acc, char v) {
      |                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   16 |                        acc.push_back(v);
      |                        ~~~~~~~~~~~~~~~~~
   17 |                        return acc;
      |                        ~~~~~~~~~~~
   18 |                    }) == "abc");
      |                    ~~~^~~~~~~~
<source>:18:32:   in 'constexpr' expansion of 'fold(T, I, S, F) [with T =
std::__cxx11::basic_string<char>; I = const char*; S = const char*; F =
<lambda(std::string, char)>](std::begin<const char, 3>(v), std::end<const char,
3>(v), (<lambda closure object><lambda(std::string, char)>(),
<lambda(std::string, char)>()))'
<source>:18:32:   in 'constexpr' expansion of
'std::__cxx11::basic_string<char>((* &
std::move<__cxx11::basic_string<char>&>(init)))'
<source>:18:23: error: accessing 'std::__cxx11::basic_string<char>::<unnamed
union>::_M_allocated_capacity' member instead of initialized
'std::__cxx11::basic_string<char>::<unnamed union>::_M_local_buf' member in
constant expression
ASM generation compiler returned: 1