[lldb] Added assert to VerifyDecl
[lldb] Added assert to VerifyDecl

We could VerifyDecl sometimes with a nullptr. It would be nice if we
could get an actual assert here instead of triggering UB.

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@365247 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb