[clang-tidy] Fix a false positive in google-runtime-memset

google-runtime-memset no longer issues a warning if the fill char value
is known to be an invalid fill char count.

Namely, it no longer warns for these:

  memset(p, 0, 0);
  memset(p, -1, 0);

In both cases, swapping the last two args would either be useless (there is
no actual bug) or wrong (it would introduce a bug).

Patch by Matt Armstrong!

llvm-svn: 257320