This CL makes `KSErrorType` a class with a string
"hint", that by convention should be a "simpleName"
of an unresolved type.

`KSErrorTypeClassDeclaration` is also no longer a singleton
and references a corresponding `KSErrorType`.

The implementation consists of extracting the available
info on the unresolved type on the best effort basis.

Allow KSErrorType to receive a message as additional
piece of information on why a valid type can't be provided.

Cover `replace`/`asType` cases with a utility
function `errorTypeOnInconsistentArguments` to
get consistent error messages across implementations.

Tweak KSTypeImpl.isError detection in AA, move TODO there.