[DTLTO] Refactor the DTLTO code. (#192629)
DTLTO implementation code has been moved from `llvm/lib/LTO/` to
`llvm/lib/DTLTO/`. This refactor does not change any externally visible
behavior, so existing DTLTO tests and documentation remain valid. The
move was done to reduce code duplication, improve maintainability, and
make it easier to adopt future performance improvements.
