[analyzer] When creating a trimmed graph, preserve whether a node is a sink.

This is important because sometimes two nodes are identical, except the
second one is a sink.

This bug has probably been around for a while, but it wouldn't have been an
issue in the old report graph algorithm. I'm ashamed to say I actually looked
at this the first time around and thought it would never be a problem...and
then didn't include an assertion to back that up.

PR15684

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178944 91177308-0d34-0410-b5e6-96231b3b80d8