Adds a simple script to merge stdout/stderr and put on labels. This is hidden to the RUN line using lit.cfg.py.
This is my solution to addressing how errors printed by the toolchain break sorting of stdout output; just put stdout first. We could also have toggles for interleaving output or such, which might help test whether we do it properly.
This also moves some previous-distributed replacement logic into lit_autoupdate_base.py: I think having that adjacent to lit.cfg.py is probably the better choice, and it reduces duplication in toolchain scripts. It happens here because I need to change the resulting commands to include the merge.
This unifies `dump-tokens` and `dump-parse-tree` so that we don't keep writing basically the same code repeatedly.
I'll need to modify the output of SemanticsIR::Print more, and may soon unify printing multiple IRs this way (particularly including the builtins SemanticsIR) but this is intended to offer a starting point.
I may eventually try to unify lit.cfg.py files, but I was thinking about whether that works in various contexts we may run in and eventually decided copying the driver/testdata/lit.cfg.py file would be the easiest solution.
Jon Ross-Perkins