[llvm] [lit] Fix some issues from --per-test-coverage (PR #65242)

[Joel E. Denny via llvm-commits]

jdenny-ornl wrote:

>     3. Rewrite this PR's title and commit log to explain what it does do: clean up the `%dbg` expansion implementation, and make `--per-test-coverage` work for lit's internal shell.

I just modified this PR accordingly and squashed previous commits into a single commit.

In the new commit log, I've summarized the windows `cmd` situation, which this PR improves but does not finish repairing.  That support can be fully repaired or discarded in a separate PR after we have more information on lit users' current needs.

I've also incorporated the latest changes from main into this PR.  To do so, I rebased and force-pushed (instead of merging), following the advice I most recently saw from various discussions about LLVM PRs.

The resulting diff is different than before, primarily because of all my lit patches I had to revert recently.  I also extended the testing.  It's probably best if reviewers look over things again before I try to land this.

This patch has code cleanup that is helpful for my other lit patches, and it doesn't seem to be controversial, so I'm now planning to land it first.

https://github.com/llvm/llvm-project/pull/65242