[llvm] [lit] Echo full RUN lines in case of external shells (PR #66408)

[Joel E. Denny via llvm-commits]

jdenny-ornl wrote:

The current PR replaces [PR #65267](https://github.com/llvm/llvm-project/pull/65267) but has some notable differences:

- I have rebased it onto the most recent versions of [D154984](https://reviews.llvm.org/D154984) and [D156954](https://reviews.llvm.org/D156954), which are based on recent main.
- I have folded in the [fix for the bot failures](https://github.com/llvm/llvm-project/pull/65267#issuecomment-1709171870) that required me to revert the original PR.  I also added a lit test for that case.
- I added a solution for suppressing duplicate printing of RUN lines (once for tracing the associated echo command, and once for the output of that echo command).  It's [proposal 3 in the RFC comment that addresses this issue](https://discourse.llvm.org/t/rfc-improving-lits-debug-output/72839/44?u=jdenny-ornl).  I decided against proposal 1 because later comments expressed concern about how much info would be lost under it.
- I applied python formatting suggestions from darker.

https://github.com/llvm/llvm-project/pull/66408