Allow some BugReports to opt-out of PathDiagnostic callstack pruning until we have...

Allow some BugReports to opt-out of PathDiagnostic callstack pruning until we have significantly
improved the pruning heuristics.  The current heuristics are pretty good, but they make diagnostics
for uninitialized variables warnings particularly useless in some cases.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@157734 91177308-0d34-0410-b5e6-96231b3b80d8