[CFLAA] Propagate StratifiedAttrs in interproc. analysis.

This patch also has a refactor that kills StratifiedAttr, and leaves us
with StratifiedAttrs, because having both was mildly redundant.

This patch makes us correctly handle stratified attributes when doing
interprocedural analysis. It also adds another attribute, AttrCaller,
which acts like AttrUnknown. We can filter out AttrCaller values when
during interprocedural analysis, since the caller should have
information about what arguments it's passing to its callee.

Patch by Jia Chen.

Differential Revision: http://reviews.llvm.org/D21645

llvm-svn: 273636