[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