[Bug c/77817] -Wimplicit-fallthrough: cpp directive renders FALLTHRU comment ineffective

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77817

--- Comment #10 from Markus Trippelsdorf <trippels at gcc dot gnu.org> ---
The testcase from PR77955:

markus@x4 /tmp % cat fall.c
void bar(int);

void foo(int i) {
  switch (i) {
  case 1: {
    bar(1);
    // fall-through
  }
  case 2:
    bar(2);
  default:
    break;
  }
}
markus@x4 /tmp % gcc -Wimplicit-fallthrough=1 -c fall.c
fall.c: In function ‘foo’:
fall.c:6:5: warning: this statement may fall through [-Wimplicit-fallthrough=]
     bar(1);
     ^~~~~~
fall.c:9:3: note: here
   case 2:
   ^~~~

Two issues; the first is the bogus warning, the second that we don't
warn for the case 2 fall-through.