Two problems:
1) We were tearing down the type checker too early, before all
relevant decls in other files had been type checked, so there
was no LazyResolver available for use in the ASTContext.
This might explain the crashes coming through
diagnoseUnintendedObjCMethodOverrides().
2) When a lazy resolver was set in the ASTContext, we were not
using it in the case where nullptr was passed in as the
'resolver' parameter to inheritsSuperclassInitializers().
This might fix the crashes where we were coming in from other
code paths, but I'm less confident about this case.
Possibly fixes <rdar://problem/29043074>, <rdar://problem/30976765>,
<rdar://problem/31122590>, and <rdar://problem/31286627>, and the
many other similar-looking dupes.
