[clang][Driver] Update/cleanup LTO logic to ensure that the last lto argument is honored

- Make flto an alias of flto=full.
- Make foffload-lto an alias of foffload-lto=full.
- Make flto_EQ_jobserver, flto_EQ_auto aliases of flto=full,
  since they are being treated as full lto right now.
- Clean up the code for parseLTOMode and setLTOMode.
- Replace uses of OPT_flto with OPT_flto_EQ since they alias now.

Differential Revision: https://reviews.llvm.org/D108881

Change-Id: I5d867db83a680434fba5c8d85c9a83135d3b81ee