[Attributor] Collect memory accesses with their respective kind and location

In addition to a single bit per memory locations, e.g., globals and
arguments, we now collect more information about the actual accesses,
e.g., what instruction caused it, was it a read/write/read+write, and
what the underlying base pointer was. Follow up patches will make
explicit use of this.

Reviewed By: uenoku

Differential Revision: https://reviews.llvm.org/D73527