Handle function drgn type when enumerating types

[tyroguru]

Summary

Handle function types when enumerating types.

Test plan

A test needs adding.

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (9e2b48d) 68.70% compared to head (86e97e7) 68.68%. Report is 1 commits behind head on main.

Files	Patch %	Lines
oi/type_graph/DrgnParser.cpp	0.00%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #422 +/- ## ========================================== - Coverage 68.70% 68.68% -0.03% ========================================== Files 119 119 Lines 11642 11647 +5 Branches 1920 1920 ========================================== + Hits 7999 8000 +1 - Misses 2670 2674 +4 Partials 973 973 ```

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[JakeHillion]

I'm wondering if this change makes total sense - we're taking in a function type which is explicitly unsized, and stating that it's a sized type StubbedPointer. I wonder if it would make more sense to immediately return an Incomplete from this switch case, along with a depth decrement. That would work if the goal is to prevent hitting the catch statement. I could be missing something though!

I wrote this bit of code which I think neatens up the depth decrement, if we do decrement in this specific switch case it would make sense to use something like this: https://github.com/JakeHillion/object-introspection/blob/6be7e7708bfd69c75faa46d42df81a79734eab46/oi/type_graph/ClangTypeParser.cpp#L45-L55

[ajor]

Finding a test case for this proved difficult. Putting this on hold until we hit this issue in production again.