[analyzer] Implement basic path diagnostic pruning based on "interesting" symbols and regions.

Essentially, a bug centers around a story for various symbols and regions.  We should only include
the path diagnostic events that relate to those symbols and regions.

The pruning is done by associating a set of interesting symbols and regions with a BugReporter, which
can be modified at BugReport creation or by BugReporterVisitors.

This patch reduces the diagnostics emitted in several of our test cases.  I've vetted these as
having desired behavior.  The only regression is a missing null check diagnostic for the return
value of realloc() in test/Analysis/malloc-plist.c.  This will require some investigation to fix,
and I have added a FIXME to the test case.

llvm-svn: 152361