tweak non-null check to put the caret on the function, but underline the

tweak non-null check to put the caret on the function, but underline the
argument.  This avoids the argument from being silenced when the argument is
the NULL macro, which is defined in a system header.  This also makes the output
a bit nicer, e.g.:

t.c:8:3: warning: null passed to a callee which requires a non-null argument
  func1(NULL, cp2, i1);
  ^     ~~~~

vs something like:

t.c:8:10: warning: argument is null where non-null is required
  func1(NULL, cp2, i1);
        ^

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@72393 91177308-0d34-0410-b5e6-96231b3b80d8