Format strings: correct signedness if already correcting width (%d,%u).

Format strings: correct signedness if already correcting width (%d,%u).

It is valid to do this:
  printf("%u", (int)x);

But if we see this:
  printf("%lu", (int)x);

...our fixit should suggest %d, not %u.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@172739 91177308-0d34-0410-b5e6-96231b3b80d8