LLDB currently crashes when the super-class of an Objective-C type can't
be completed (i.e., has no definition). For Foundation types such as
`NSObject`, the debug-info would usually only contain forward
declarations. The definitions live in the Clang module `.pcm` files. But
if the source of the definition fails to be loaded (e.g., if we just
delete the module cache), then we can no longer guarantee that the
super-class has a definition. This breaks a key Clang invariant, which
requires base-classes to have definitions by the time we try to lay them
out. This patch adds an XFAILed test for such scenario.
rdar://168440264
230f6ab1b8