[Sema] Add a new warning, -Wmemset-transposed-args

This diagnoses calls to memset that have the second and third arguments
transposed, for example:

  memset(buf, sizeof(buf), 0);

This is done by checking if the third argument is a literal 0, or if the second
is a sizeof expression (and the third isn't). The first check is also done for
calls to bzero.

Differential revision: https://reviews.llvm.org/D49112

llvm-svn: 337470