-Wint-conversion as errors seems doable for GCC 14

[Florian Weimer via Gcc]

I completed a Fedora rawhide rebuild with an instrumented GCC (~14,500
packages).  156 packages failed to build with a logged -Wint-conversion
error.  This number is much lower than what I expected, and I think we
should include -Wint-conversion in the GCC 14 changes.

My instrumentation isn't very good and has false positives due to
cascading errors, such as this example from Emacs:

conftest.c: In function 'main':
conftest.c:246:21: error: passing argument 1 of 'pthread_setname_np' makes 
integer from pointer without a cast
  246 | pthread_setname_np ("a");
      |                     ^~~
      |                     |
      |                     char *
In file included from conftest.c:242:
/usr/include/pthread.h:463:42: note: expected 'pthread_t' {aka 'long unsigned 
int'} but argument is of type 'char *'
  463 | extern int pthread_setname_np (pthread_t __target_thread, const char 
*__name)
      |                                ~~~~~~~~~~^~~~~~~~~~~~~~~
conftest.c:246:1: error: too few arguments to function 'pthread_setname_np'
  246 | pthread_setname_np ("a");
      | ^~~~~~~~~~~~~~~~~~
/usr/include/pthread.h:463:12: note: declared here
  463 | extern int pthread_setname_np (pthread_t __target_thread, const char 
*__name)
      |            ^~~~~~~~~~~~~~~~~~

The error at column 21 is logged, even though it is harmless because the
compilation can never succeed because of the wrong argument count.

That's why I think the number of failing builds 156 is really quite low,
and we should be able to manage on the Fedora side.  I'll eventually do
another rebuild with better instrumentation (one that also handles
waivers for implicitly declared functions; currently we get errors from
those because their type is still treated as int internally; maybe I
should switch to error_mark_node for that).

Thanks,
Florian