[RemoveDIs] Load into new debug info format by default in llvm-lto and llvm-lto2 (#86271)

Directly load all bitcode into the new debug info format in `llvm-lto`
and `llvm-lto2`. This means that new-mode bitcode no longer round-trips
back to old-mode after parsing, and that old-mode bitcode gets
auto-upgraded to new-mode debug info (which is the current in-memory
default in LLVM).