[lldb/Reproducers] Add top-level-target check-lldb-reproducers

This adds a new target `check-lldb-reproducers` that replaces the old
`check-lldb-repro`. The latter would only run the shell tests, while
`check-lldb-reproducers` includes the API tests as well. The new target
will be used on GreenDragon.

It's still possible to run just the shell tests with reproducers,
although now that requires crafting the lit invocation yourself. The
parameters haven't changed and are the shame for the API and shell
tests:

  --param lldb-run-with-repro=capture
  --param lldb-run-with-repro=replay

This patch also updates the reproducer documentation.