The %diff_plist lit substitution invokes diff with a non-portable -I option. The intended effect can be achieved by normalizing the inputs to diff beforehand. Such normalization can be done with grep -Ev, which is also used by other tests.
This patch applies the change (adjusted for review comments) described in http://lists.llvm.org/pipermail/cfe-dev/2019-April/061904.html mechanically to the cases where the output file is piped to %diff_plist via tail. %diff_plist is then, being unused, removed.
The changes were applied via a script.