[Bug c/116082] New: -Wunterminated-string-initialization should not warn about strings that end with "\0" but should warn still for -Wc++-compat

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116082

            Bug ID: 116082
           Summary: -Wunterminated-string-initialization should not warn
                    about strings that end with "\0" but should warn still
                    for -Wc++-compat
           Product: gcc
           Version: 15.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

This was reduced from the linux kernel sources, they add a "\0" to make sure it
is always ends in a null character.
-Wunterminated-string-initialization warns still in this case:
```
char t[31] = "CHARGE_CONTROL_START_THRESHOLD""\0";
```

But it is not unterminated but rather it is just it is 31 character string with
a null character as the end. Note we should still warn for -Wc++-compat since
that is still invalid C++ code.