[analyzer] Look through OpaqueValueExprs when tracking a nil value.

This allows us to show /why/ a particular object is nil, even when it is
wrapped in an OpaqueValueExpr.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@164445 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed