Implement pass_defines (fixed)

[oriceemple]

No description provided.

[regehr]

continuing the discussion, my opinion is that depending on clang is perfectly OK since we already expect clang-format to exist

[regehr]

also, as Eric says, squashing at the end is the preferred workflow. github also has an auto-squash on merge feature that I haven't tried yet.

[regehr]

It's an interesting approach. Can you share some examples where this approach works but where it wasn't possible to preprocess before running C-Reduce?

[oriceemple]

I can't give exact details, but I can explain where pass_defines helps me:

1. It allows reducing work in the pass_unifdef phase.
2. When testing code that uses precompiled headers, we want to continue running through the precompiled header path instead of forcing everything to go in the cpp.
3. A reduce that depends on file-specific behaviour, such as #pragma GCC system_header will misbehave after preprocessing.