Restore "[LTO/WPD] Enable aggressive WPD under LTO option"

This restores 59733525 (D71913), along
with bot fix 19c76989.

The bot failure should be fixed by D73418, committed as
af954e44.

I also added a fix for non-x86 bot failures by requiring x86 in new test
lld/test/ELF/lto/devirt_vcall_vis_public.ll.