[llvm] [Dexter] Make lldb-dap _post_step_hook more stable (PR #157663)

[Orlando Cazalet-Hyams via llvm-commits]

OCHyams wrote:

@JDevlieghere do you know if there is someone that might be able to help me understand why we're seeing this output from lldb-dap?

To clarify, the `KeyError` I mentioned is in dexter, but the important question and part I don't understand is why `lldb-dap` occasionally (1 in 15 buildbot runs) doesn't give us a "module" event (for `libc.so.6`) that it otherwise usually does. 

https://github.com/llvm/llvm-project/pull/157663