Revert "[Clang] [Diagnostics] Simplify filenames that contain '..'" (#148367)
Revert llvm/llvm-project#143520 for now since it’s causing issues for
people who are using symlinks and prefer to preserve the original path
(i.e. looks like we’ll have to make this configurable after all; I just
need to figure out how to pass `-no-canonical-prefixes` down through the
driver); I’m planning to refactor this a bit and reland it in a few
days.