Handle insertion fix-its when removing incompatible errors by introducting a new EventType ET_Insert
This has lower prioirty than End events, but higher than begin.
Idea being If an insert is at the same place as a begin event, the insert should be processed first to reduce unnecessary conflicts.
Likewise if its at the same place as an end event, process the end event first for the same reason.
This also fixes https://bugs.llvm.org/show_bug.cgi?id=46511.