[ThinLTO] Reduce pipeline clang test to avoid churn from LLVM changes

This test was added in D72538, along with multiple LLVM pipeline tests,
to ensure that distributed ThinLTO backends invoked via clang set up the
expected ThinLTO optimization pipeline. However, this introduces churn
to clang tests from LLVM pipeline changes (see recent comment in that
patch).

Since the full pipeline setup is tested by LLVM, I have changed this
test to simply look for a single pass that is only invoked during LTO
backends, to make sure that clang is provoking the an LTO backend
pipeline setup.