[ELF] Rename --[no-]lto-new-pass-manager to --[no-]lto-legacy-pass-manager

Normally we should not delete options. However, the Clang driver passes
`-plugin-opt={new,legacy}-pass-manager` instead of
`--[no-]lto-legacy-pass-manager` (`-plugin-opt=new-pass-manager` has been used
since 7.0), and it is unlikely anyone will use the `--lto-*` style options directly.

So let's rename them to be consistent with the Clang driver option names.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D92988