[lit] Echo full RUN lines in case of external shells (#65267)

Before <https://reviews.llvm.org/D154984> and
<https://reviews.llvm.org/D156954>, lit reported full RUN lines in a
`Script:` section. Now, in the case of lit's internal shell, it's the
execution trace that includes them. However, if lit is configured to use
an external shell (e.g., bash, windows `cmd`), they aren't reported at
all.

A fix was requested at the following:

* <https://reviews.llvm.org/D154984#4627605>
*
<https://discourse.llvm.org/t/rfc-improving-lits-debug-output/72839/35?u=jdenny-ornl>

This patch does not correctly address the case when the external shell
is windows `cmd`. As discussed at
<https://github.com/llvm/llvm-project/pull/65242>, it's not clear
whether that's a use case that people still care about, and it seems to
be generally broken anyway.