[analyzer] Ensure that the node NilReceiverBRVisitor is looking for is not reclaimed

The visitor should look for the PreStmt node as the receiver is nil in the PreStmt and this is the node. Also, tag the nil
receiver nodes with a special tag for consistency.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178152 91177308-0d34-0410-b5e6-96231b3b80d8
[analyzer] Cleanup: only get the PostStmt when we need the underlying Stmt + comment

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178153 91177308-0d34-0410-b5e6-96231b3b80d8