Add support for __randomize_layout

[connorkuehl]

Is your feature request related to a problem? Please describe.
Our minimum viable product requires that application programmers opt-in to have their structures laid out randomly.

Describe the solution you'd like
When Clang hands control to our plugin, we will check if the structure has the __randomize_layout attribute attached to it. If not, we will return false; and allow Clang to lay it out. If it does have the attribute, we will lay it out.

[tim-pugh]

This will depend on llvm#100 first, but this is the much more important issue.

I would finish off llvm#100, branch and then develop this patch. We need to test that our plugin works with Clang V9 first.

[connorkuehl]

Closed by the following commits:

9b0f3de
4ff065b