[lldb] [llvm] [DWARF] Add DW_OP_LLVM_NVIDIA_mux vendor extension as a subop of DW_OP_LLVM_user (PR #216314)

[Matt Davis via llvm-commits]

enferex wrote:

@echristo I added two more unit tests per your offline suggestion, and also adjusted the comment to DWARFExpression::Operation. Thank you for that feedback.

https://github.com/llvm/llvm-project/pull/216314