Remove a bunch of obscene double-buffering of BugReports in the retain/release

Remove a bunch of obscene double-buffering of BugReports in the retain/release
checker. This was previously needed because BugReport objects were previously
allocated on the stack and not owned by BugReporter. Now we can just issue them
on the fly. This change was motivated because we were seeing some weird cases
where some really long paths would get issued for bugs (particularly leaks)
because of some double-caching.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@63840 91177308-0d34-0410-b5e6-96231b3b80d8