[IR] Introduce a non-integral pointer type

Summary:
This change adds a `ni` specifier in the `datalayout` string to denote
pointers in some given address spaces as "non-integral", and adds some
typing rules around these special pointers.

Reviewers: majnemer, chandlerc, atrick, dberlin, eli.friedman, tstellarAMD, arsenm

Subscribers: arsenm, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D22488

llvm-svn: 277085