[CFLAA] Add AttrEscaped, remove bit twiddling functions.

This patch does a few things:

- Unifies AttrAll and AttrUnknown (since they were used for more or less
  the same purpose anyway).

- Introduces AttrEscaped, an attribute that notes that a value escapes
  our analysis for a given set, but not that an unknown value flows into
  said set.

- Removes functions that take bit indices, since we also had functions
  that took bitsets, and the use of both (with similar names) was
  unclear and bug-prone.

Patch by Jia Chen.

Differential Revision: http://reviews.llvm.org/D21000

llvm-svn: 272040