[analyzer] Fix use-after-free in HandleTranslationUnit.

A patch by Dmitri Gribenko!

The attached patch fixes a use-after-free in AnalysisConsumer::HandleTranslationUnit.  The problem is that
BugReporter's destructor runs after AnalysisManager has been already
deleted.  The fix introduces a scope to force correct destruction
order.

A crash happens only when reports have been added in AnalysisConsumer::HandleTranslationUnit's BugReporter. We don't have such checkers in clang so no test.

llvm-svn: 147732