[Assignment Tracking] Follow up for D147129 - fix broken clang tests

D147129 causes the assignment tracking pass to skip optnone functions, which
ends up overriding the "-fexperimental-assignment-tracking=forced" option at
O0. Add -disable-O0-optnone to the tests so that optnone doesn't interfere.

Buildbot example: https://lab.llvm.org/buildbot/#/builders/216/builds/18986