[CodeGen] Store ElementType in Address

Explicitly track the pointer element type in Address, rather than
deriving it from the pointer type, which will no longer be possible
with opaque pointers. This just adds the basic facility, for now
everything is still going through the deprecated constructors.

I had to adjust one place in the LValue implementation to satisfy
the new assertions: Global registers are represented as a
MetadataAsValue, which does not have a pointer type. We should
avoid using Address in this case.

This implements a part of D103465.

Differential Revision: https://reviews.llvm.org/D115725