We're implementing funclet-compatible code generation for Obj-C
exceptions when using the MSVC ABI. The idea is that the Obj-C runtime
will wrap Obj-C exceptions inside C++ exceptions, which allows for
interoperability with C++ exceptions (for Obj-C++) and zero-cost
exceptions. This is the approach taken by e.g. WinObjC, and I believe it
to be the best approach for Obj-C exceptions in the MSVC ABI.
The first step is emitting proper RTTI for Obj-C exception types. Since
we're wrapping Obj-C exceptions in C++ exceptions, the RTTI should be
identical, barring the name of the RTTI variable (OBJC_EHTYPE_$_*).
Since the MSVC ABI does not easily allow for cross-DLL data references
from within other data, we instead emit the RTTI locally wherever
needed, which is also how C++ RTTI works on that ABI.
Follow-up diffs will add code generation support for @try itself, but
I'm splitting it up to get early feedback and make review more
Worked on with Saleem Abdulrasool <email@example.com>.