Improved tests for nested structs when anonymous structs are involved.
- New tests can fail on OS/X and Linux, and illustrate that the compiler
used to generate the DWARF can result in lldb providing clang with an
external AST source that doesn't describe all required struct fields.

- Also includes test coverage for expressions with structs that do work on Linux- Also includes a test for dereferencing a null pointer to a struct, which works on OS/X rather than complaining, and complains on Linux with an upstream error that is really a side issue.

Thanks to Samuel, Andy and Daniel for their input.