[analyzer] Add stop-gap patch to prevent assertion failure when analyzing LLVM codebase.

This potentially reduces a performance optimization of throwing away
PreStmtPurgeDeadSymbols nodes.  I'll investigate the performance impact
soon and see if we need something better.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176149 91177308-0d34-0410-b5e6-96231b3b80d8