[llvm] [Dexter] Implement DexStepFunction and DexContinue (PR #152721)

[Orlando Cazalet-Hyams via llvm-commits]

OCHyams wrote:

I've added two commits to fix test failures -
* The 1st fixes a sync issue with the receiver thread (previously it signalled it had got a response before fully handling it itself, leading to other threads reading incomplete debugger state)
* The 2nd filters out breakpoint IDs that we didn't set from dexter. LLDB-DAP is sometimes returning negative breakpoint IDs

@SLTozer does this still look good?

https://github.com/llvm/llvm-project/pull/152721