[analyzer] Fix crash in RetainCountChecker::checkEndFunction

The class BodyFarm creates bodies for
OSAtomicCompareAndSwap*, objc_atomicCompareAndSwap*, dispatch_sync*, dispatch_once*
and for them the flag isBodyAutosynthesized is set to true.

This diff
1. makes AnalysisConsumer::HandleCode skip the autosynthesized code
2. replaces assert(LCtx->getParent()) in RetainCountChecker::checkEndFunction
by assert(!LCtx->inTopFrame()) (minor cleanup)

Test plan: make -j8 check-clang-analysis

Differential revision: https://reviews.llvm.org/D24792

llvm-svn: 282293