[C][Sema] -Wformat misleading idempotent FixItHint for scanning into const

[whisperity]

The following code:

#include <stdio.h>
#include <stdlib.h>

void parse(char* in) {
    const char* B = (char*) malloc(42);
    sscanf(in, "%s", B);
}

<source>:7:23: warning: format specifies type 'char *' but the argument has type 'const char *' [-Wformat]
    7 |     sscanf(pos, "%s", B);
      |                  ~~   ^
      |                  %s

https://godbolt.org/z/Gnqvq4Kvd

produces a -Wformat warning that has a FixItHint attached, which does not actually change anything related to the code. Such a warning is a problem in interactive editors where the editor shows "fix available", and after applying the fix, the warning is still there.