[NFC][LLVM][CodeGen] Move LiveDebugVariables.h into llvm/include/llvm/CodeGen (#88374)

This patch make `LiveDebugVariables` can be used by passes outside of
`lib/CodeGen`.

If we run a pass that occurs between the split register allocation pass
without preserving this pass, it will be freed and recomputed until it
encounters the next pass that needs LiveDebugVariables.

However, `LiveDebugVariables` will raise an assertion due to the pass
being freed without emitting a debug value.

This is reason we need `LiveDebugVariables` to be available for passes
outside of lib/Codegen.