[analyzer] Collect stats on the max # of bug reports in an equivalence class.

We discovered that sqlite3.c currently has 2600 reports in a single
equivalence class; it would be good to know if this is a recent
development or what.

(For the curious, the different reports in an equivalence class represent
the same bug found along different paths. When we're suppressing false
positives, we need to go through /every/ path to make sure there isn't a
valid path to a bug. This is a flaw in our after-the-fact suppression,
made worse by the fact that that function isn't particularly optimized.)

llvm-svn: 177188