[CodeGen] Add a flag to `Address` and `Lvalue` that is used to keep

track of whether the pointer is known not to be null

The flag will be used for the arm64e work we plan to upstream in the
future (see https://lists.llvm.org/pipermail/llvm-dev/2019-October/136091.html).
Currently the flag has no effect on code generation.

Differential Revision: https://reviews.llvm.org/D142584