[llvm] [Dexter] Update Dexter lit substitutions, making lldb-dap the default (PR #155838)

[Orlando Cazalet-Hyams via llvm-commits]

https://github.com/OCHyams commented:

We discussed moving away from supporting non-dap debuggers (& deprecating non-dap debugger controllers) , so with that in mind my preference would be to not fall back to LLDB (that slightly simplifies the testing matrix and aligns with our intentions). Wdyt?

https://github.com/llvm/llvm-project/pull/155838