Previously ReflectionTypes.find returned an error class in case a class
is not found in the module dependencies. The problem with this approach
is that each call site should call ErrorUtils.isError on the result and
report an error if needed, in order to stop this type from reaching the
codegen, which can't handle error types.
Now we create a MockClassDescriptor instance instead. It's not an error
class, so it'll be handled correctly in the codegen. Also its scope is
empty and errors are reported on any non-trivial usage (see
MissingDependencyClassChecker), so this approach is not worse than error
classes
#KT-16484 Fixed
Ilya Gorbunov