[analyzer] Separate graph trimming from creating the single-path graph.

When we generate a path diagnostic for a bug report, we have to take the
full ExplodedGraph and limit it down to a single path. We do this in two
steps: "trimming", which limits the graph to all the paths that lead to
this particular bug, and "creating the report graph", which finds the
shortest path in the trimmed path to any error node.

With BugReporterVisitor false positive suppression, this becomes more
expensive: it's possible for some paths through the trimmed graph to be
invalid (i.e. likely false positives) but others to be valid. Therefore
we have to run the visitors over each path in the graph until we find one
that is valid, or until we've ruled them all out. This can become quite
expensive.

This commit separates out graph trimming from creating the report graph,
performing the first only once per bug equivalence class and the second
once per bug report. It also cleans up that portion of the code by
introducing some wrapper classes.

This seems to recover most of the performance regression described in my
last commit.

<rdar://problem/13433687>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177216 91177308-0d34-0410-b5e6-96231b3b80d8

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