Fix: CFLAA -- Mark no-args returns as unknown

Fix: CFLAA -- Mark no-args returns as unknown

Prior to this patch, we hadn't been marking StratifiedSets with the
appropriate StratifiedAttrs when handling the result of no-args call
instructions. This caused us to report NoAlias when handed, for
example, an escaped alloca and a result from an opaque function. Now we
properly mark the return value of said functions.

Thanks again to Chandler, Richard, and Nick for pinging me about this.

Differential review: http://reviews.llvm.org/D12408

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246240 91177308-0d34-0410-b5e6-96231b3b80d8