post-rebase: Do not put "(Ast)" suffix in error msg unless passed `-Z borrowck-mir`.
(But unconditionally include "(Mir)" suffix for mir-borrowck errors.)
After repatriating error explanations to the global registry, some lurking
doctest failures surfaced and needed to be chased down. Sadly, a few doctests
needed to be ignored due to a not-yet-understood regression in the doctest
`compile_fail` functionality (filed #43707).
