[CFLAA] Tag arguments as escaped instead of unknown.

This patch also includes some refactoring.

Prior to this patch, we tagged all CFLAA attributes as unknown. This is
suboptimal, since it meant that any Value used as an argument would be
considered to alias any other Value that existed.

Now that we have the machinery to tag sets below the set for an
arbitrary value with attributes, it's okay to be less conservative with
arguments. (Specifically, we still tag the set under an argument with
unknown).

Patch by Jia Chen.

Differential Revision: http://reviews.llvm.org/D21262

llvm-svn: 272690