[clang][analyzer] StreamChecker: Model getc, vfscanf, putc, vfprintf (#82476)

Model `getc` and `putc` as equivalent to `fgetc` and `fputc` respectively.

Model `vfscanf` and `vfprintf` as `fscanf` and `fprintf`, except that
`vfscanf` can not invalidate the parameters due to the indirection via a
`va_list`. Nevertheless, we can still track EOF and errors as for `fscanf`.