[llvm] [Dexter] Only increment step index if we record the step (PR #156832)

[Orlando Cazalet-Hyams via llvm-commits]

OCHyams wrote:

@aprantl Looks like with this change on top of Stephen's, the `cross-project-tests :: debuginfo-tests/dexter/feature_tests/commands/control/dex-continue.cpp` failure is fixed. The bot is still red, but the dexter test is passing from https://green.lab.llvm.org/job/llvm.org/view/LLDB/job/as-lldb-cmake/32551/consoleText

Thanks for your patience while we dealt with those two snags

https://github.com/llvm/llvm-project/pull/156832