[gardening] Make SILUndef::get(..., SILModule &) the impl method and the one that takes the SILModule * the helper method instead of vis-a-versa.
We already assume in the given static method that the module is a non-null
pointer. So rather than us indirecting a ref to a pointer and then dereferencing
the pointer, we instead use the ref in the module implementation and dereference
the pointer.
rdar://46294760