[PM/AA] Rework the names and comments in AliasSetTracker to more

accurately describe what is being tracked.

While these two enums do track mod/ref information and aliasing
information, they don't represent the exact same things as either the
mod/ref enums or the alias result enum in AA. They're definitions are
dominated by the structure of their lattice and the bit's various
semantics. This patch just calls them what they are and tries to spell
out usefully distinct names for these things.

This will clear the path for using a raw unscoped enum to represent some
of these concepts across LLVM's analysis library.

No functionality changed here.

Differential Revision: http://reviews.llvm.org/D10494

llvm-svn: 240254