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".
Adds a new frontend option
"-experimental-allow-module-with-compiler-errors". If any compilation
errors occur while generating the .swiftmodule, this mode will skip SIL
entirely and only serialize the (likey invalid) AST.
This existence of this option during generation is serialized into the
resulting .swiftmodule. Errors found in deserialization are only allowed
if it is set.
Primarily intended for IDE requests (eg. indexing and code completion)
to ensure robust cross-module results, despite possible errors.
Resolves rdar://69815975
