Lazy loading checked if the ClangDecl was hidden, but loading all
members did not. Let's make loadAllMembers() behave like the lazy
path, and fix some of the mock SDKs in the test suite.
There were a few problems here with subclasses of Objective-C classes.
Use the InstanceStart field from rodata to correctly lay out instance
variables, and verify the results match with dynamic and static layout.
Better fix for <rdar://problem/27932061>.