In preparation for removing UnresolvedType in favor of ErrorType,
start printing ErrorType as `_` unless we've enabled debug printing.
`<<error type>>` should never be presented to the user, instead `_`
now just consistently means "unknown type".
Use the `%target-swift-5.X-abi-triple` substitutions to compile the tests for
deployment to the minimum OS versions required for the APIs used in the tests,
instead of disabling availability checking.
Some editors use diagnostics from SourceKit to replace build issues. This causes issues if the diagnostics from SourceKit are formatted differently than the build issues. Make sure they are rendered the same way, removing most uses of `DiagnosticsEditorMode`.
To do so, always emit the `add stubs for conformance` note (which previously was only emitted in editor mode) and remove all `; add <something>` suffixes from notes that state which requirements are missing.
rdar://129283608
