[utils] change default nameless value to "TMP"

This is effectively reverting rGbfdc2552664d to avoid test churn
while we figure out a better way forward.

We at least salvage the warning on name conflict from that patch
though.

If we change the default string again, we may want to mass update
tests at the same time. Alternatively, we could live with the poor
naming if we change -instnamer.

This also adds a test to LLVM as suggested in the post-commit
review. There's a clang test that is also affected. That seems
like a layering violation, but I have not looked at fixing that yet.

Differential Revision: https://reviews.llvm.org/D80584