Use standard preprocessor definitions in codegen tests

Certain language features — say, `offsetof` — rely on preprocessor for their implementation.

It really does make sense to write tests for these features using the same preprocessor definitions we use in the standard library.

But currently, there's no good way of achieving this: the standard includes are unavailable in the `CodeGenTestBase`-derived tests.

Let's allow the tests to use standard includes (either unblock that in `CodeGenTestBase` or create a new test base class for that), and then enable the tests marked with `TODO[#390]`.