[Bug c++/98499] [11 Regression] Possibly bad std::string initialization in constructors

["slyfox at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98499

--- Comment #1 from Sergei Trofimovich <slyfox at gcc dot gnu.org> ---
Managed to get rid of external <string> dependency:

```
struct string {
  char * _M_buf;
  // local store
  char _M_local_buf[16];

  string(const string &__str) : _M_buf(_M_local_buf) {}

  string(const char *__s) : _M_buf(_M_local_buf) {}

  ~string() {
    if (_M_buf != _M_local_buf)
      __builtin_trap();
  }

  // not defined
  string();
};

// main.cc

__attribute__((noinline)) static string dir_name() { return "c"; }
__attribute__((noinline)) static string make_canonical_path(string path) {
  return path;
}
class Importer {
public:
  string imp_path;
  string ctx_path;
  string base_path;

public:
  __attribute__((noinline)) Importer(string imp_path, string ctx_path)
      : imp_path(make_canonical_path(imp_path)),
        ctx_path(make_canonical_path(ctx_path)), base_path(dir_name()) {}
};
struct Include {
  Include(const Importer &imp) {}
};
int main() { const Include &inc = {{"a", "b"}}; }
```

Note: all our string() constructors shoudl have `_M_buf == _M_local_buf`
invariant. But gcc-11 generates always-crashing program:

```
; g++-11.0.0 -O2 -std=c++11 -S main.cc
main:
        .cfi_startproc
        subq    $152, %rsp
        .cfi_def_cfa_offset 160
        movq    %fs:40, %rax
        movq    %rax, 136(%rsp)
        xorl    %eax, %eax
        leaq    8(%rsp), %rax
        movq    %rsp, %rsi
        leaq    32(%rsp), %rdx
        movq    %rax, (%rsp)
        leaq    64(%rsp), %rdi
        leaq    40(%rsp), %rax
        movq    %rax, 32(%rsp)
        call    _ZN8ImporterC1E6stringS0_
        ud2
```