[analyzer] Fix bug in isInevitablySinking

If the non-sink report is generated at the exit node, it will be
suppressed by the current functionality in isInevitablySinking, as it
only checks the successors of the block, but not the block itself.

The bug shows up in RetainCountChecker checks.

Differential Revision: https://reviews.llvm.org/D52284

llvm-svn: 342766