Support disabling rewriting for specific function pointer types

[randomPoison]

For some function pointers in a library, we don't want those to be rewritten because we know the function pointers are entirely internal to the compartment. We can use IA2_IGNORE in all of the places where function addresses are taken, but the fn ptr type will still get rewritten, resulting in compiler errors. We need a general mechanism for marking function pointer types that shouldn't be rewritten at all. IA2_BEGIN_NO_WRAP seems like the right mechanism for this, but currently it only applies to direct function calls, and can't be used in the way we want.

[kkysen]

From what I can tell, I don't think we need to change the apply_to = hasType(functionType) in IA2_BEGIN_NO_WRAP. With this

https://godbolt.org/z/3qsfqYEdG

typedef void(*a_fn)(int);

IA2_BEGIN_NO_WRAP

typedef void(*b_fn)(int, int);

struct DSP1 {
    a_fn a;
    b_fn b;
};

IA2_END_NO_WRAP

struct DSP2 {
    a_fn a;
    b_fn b;
};

b_fn, DSP1::a, DSP2::b all get AnnotateAttrs with ia2_skip_wrap.

So what we need to do is the rest of what @ayrtonm outlined:

2. add an ignore_type to SourceRewriter.cpp analogous to the existing ignore_function
3. call this ignore_type somewhere in the run method in the FnPtrTypes class