[analyzer] Add stop-gap patch to prevent assertion failure when analyzing LLVM codebase.
[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: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@176149 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang