incorrect range highlight on unused value of conditional expression

[mycoboco]

void f(void)
{
    int x;

    (0)? x: x;
}

gives

foo.c:5:5: warning - expression result not used
      (0)? x: x;
      ^~~

[mycoboco]

Fixing to highlight the whole conditional expression with pin-pointing sub-expressions that are not used makes the code much more complicated than expected; it requires an additional parameter to tree_chkused() to denote a dry-run and a distinguished return value to signal that only one operand has no side effect on a dry-run.

Highlighting a sub-expression seems to be a good compromise considering other product compilers.