[CodeGen] Require use of Address::invalid() for invalid address (NFC)

This no longer allows creating an invalid Address through the regular
constructor. There were only two places that did this (AggValueSlot
and EHCleanupScope) which did this by converting a potential nullptr
into an Address. I've fixed both of these by directly storing an
Address instead.

This is intended as a bit of preliminary cleanup for D103465.

Differential Revision: https://reviews.llvm.org/D115630